[class-parse] .jmod file support

Context: dotnet#858
Context: https://stackoverflow.com/questions/44732915/why-did-java-9-introduce-the-jmod-file-format/64202720#64202720
Context: https://openjdk.java.net/projects/jigsaw/
Context: xamarin/monodroid@c9e5cbd

JDK 9 replaced the "venerable" (and huge, ~63MB) `jre/lib/rt.jar`
with a set of `.jmod` files.

Thus, as of JDK 9, there is no `.jar` file to try to parse with
`class-parse`, only `.jmod` files!

A `.jmod` file, in turn, is still a ZIP container, much like `.jar`
files, but:

 1. With a different internal directory structure, and
 2. With a custom file header.

The result of (2) is that while `unzip -l` can show the contents of
a `.jmod` file -- with a warning -- `System.IO.Compression.ZipArchive`
cannot process the file:

	% mono …/class-parse.exe $HOME/android-toolchain/jdk-11/jmods/java.base.jmod
	class-parse: Unable to read file 'java.base.jmod': Number of entries expected in End Of Central Directory does not correspond to number of entries in Central Directory.
	<api
	  api-source="class-parse" />

Update `Xamarin.Android.Tools.Bytecode.ClassPath` to support `.jmod`
files by using `PartialStream` (73096d9) to skip the first 4 bytes.

Once able to read a `.jmod` file, *warnings* appeared, a'la

	class-parse: method com/xamarin/JavaType$1MyStringList.<init>(Lcom/xamarin/JavaType;Ljava/lang/String;ILjava/lang/StringBuilder;)V:
	  Local variables array has 2 entries ('LocalVariableTableAttribute(
	      LocalVariableTableEntry(Name='this', Descriptor='Lcom/xamarin/JavaType$1MyStringList;', StartPC=0, Index=0),
	      LocalVariableTableEntry(Name='this$0', Descriptor='Lcom/xamarin/JavaType;', StartPC=0, Index=1),
	      LocalVariableTableEntry(Name='a', Descriptor='Ljava/lang/String;', StartPC=0, Index=2),
	      LocalVariableTableEntry(Name='b', Descriptor='I', StartPC=0, Index=3))'
	  ); descriptor has 3 entries!
	class-parse: method com/xamarin/JavaType$1MyStringList.<init>(Lcom/xamarin/JavaType;Ljava/lang/String;ILjava/lang/StringBuilder;)V:
	Signature ('Signature((Ljava/lang/String;I)V)') has 2 entries; Descriptor '(Lcom/xamarin/JavaType;Ljava/lang/String;ILjava/lang/StringBuilder;)V' has 3 entries!

This was a variation on the "JDK 8?" block that previously didn't
have much detail, in part because it didn't have a repro.  Now we
have a repro, based on [JDK code][0] which contains a class
declaration within a method declaration

	public List<String> staticActionWithGenerics(…) {
	    class MyStringList extends ArrayList<String> {
	        public MyStringList(String a, int b) {
	        }
	        public String get(int index) {
	            return value.toString();
	        }
	    }
	}

The deal is that `staticActionWithGenerics()` contains a `MyStringList`
class, which in turn contains a constructor with two parameters.
*However*, as far as Java bytecode is concerned, the constructor
contains *4* local variables with StartPC==0, which is what we use to
infer parameter names.

Refactor, cleanup, and otherwise rewrite huge swaths of `Methods.cs`
to get to a "happy medium" of:

  * No warnings from our unit tests, ensured by updating
    `ClassFileFixture` to have a `[SetUp]` method which sets the
	`Log.OnLog` field to a delegate which may call `Assert.Fail()`
	when invoked.  (This asserts for all non-Kotlin messages.)

  * No warnings when processing `java.base.jmod`:

        % mono bin/Debug/class-parse.exe $HOME/android-toolchain/jdk-11/jmods/java.base.jmod >/dev/null
        # no error messages

  * No warnings when processing Android API-31:

        % mono bin/Debug/class-parse.exe $HOME/android-toolchain/sdk/platforms/android-31/android.jar >/dev/null
        # no error messages

Aside: closures are *weird* and finicky.
Consider the following Java code:

	class ClosureDemo {
	    public void m(String a) {
	        class Example {
	            public Example(int x) {
	                System.out.println (a);
	            }
	        }
	    }
	}

It looks like the JNI signature for the `Example` constructor might
be `(I)V`, but isn't.  It is instead:

	(LClosureDemo;ILjava/lang/String;)V

Breaking that down:

  * `LClosureDemo;`: `Example` is an inner class, and thus has an
    implicit reference to the containing type.  OK, easy to forget.

  * `I`: the `int x` parameter.  Expected.

  * `Ljava/lang/String`: the `String a` parameter from the enclosing
    scope!  This is the closure parameter.

This does make sense.  The problem is that it's *selective*: only
variables used within `Example` become extra parameters.
If the `Example` constructor is updated to remove the
`System.out.println(a)` statement, then `a` is no longer used, and
is no longer present as a constructor parameter.

The only way I found to "reasonably" determine if a constructor
parameter was a closure parameter was by checking all fields in the
class with names starting with `val$`, and then comparing the types
of those fields to types within the enclosing method's descriptor.
I can't think of a way to avoid using `val$`. :-()

[0]: https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/stream/WhileOps.java#L334

src/Xamarin.Android.Tools.Bytecode/AttributeInfo.cs

@@ -360,6 +360,8 @@ public string InnerName {
 			}
 		}
 
+		public string OuterClassName => OuterClass?.Name?.Value;
+
 		public override string ToString ()
 		{
 			return string.Format ("InnerClass(InnerClass='{0}', OuterClass='{1}', InnerName='{2}', InnerClassAccessFlags={3})",

src/Xamarin.Android.Tools.Bytecode/ClassPath.cs

@@ -41,14 +41,22 @@ public ClassPath (string path = null)
 			Load (path);
 		}
 
-		public void Load (string jarFile)
+		public void Load (string filePath)
 		{
-			if (!IsJarFile (jarFile))
-				throw new ArgumentException ("'jarFile' is not a valid .jar file.", "jarFile");
-
-			using (var jarStream = File.OpenRead (jarFile)) { 
-				Load (jarStream);
+			if (IsJmodFile (filePath)) {
+				using (var source   = File.OpenRead (filePath)) {
+					var slice       = new PartialStream (source, 4);
+					Load (slice);
+				}
+				return;
+			}
+			if (IsJarFile (filePath)) {
+				using (var jarStream = File.OpenRead (filePath)) {
+					Load (jarStream);
+				}
+				return;
 			}
+			throw new ArgumentException ($"`{filePath}` is not a supported file format.", nameof (filePath));
 		}
 
 		public void Load (Stream jarStream, bool leaveOpen = false)
@@ -113,6 +121,23 @@ public static bool IsJarFile (string jarFile)
 			}
 		}
 
+		public static bool IsJmodFile (string jmodFile)
+		{
+			if (jmodFile == null)
+				throw new ArgumentNullException (nameof (jmodFile));
+			try {
+				var f = File.OpenRead (jmodFile);
+				var h = new byte[4];
+				if (f.Read (h, 0, h.Length) != 4) {
+					return false;
+				}
+				return h[0] == 0x4a && h[1] == 0x4d && h[2] == 0x01 && h[3] == 0x00;
+			}
+			catch (Exception) {
+				return false;
+			}
+		}
+
 		XAttribute GetApiSource ()
 		{
 			if (string.IsNullOrEmpty (ApiSource))

src/Xamarin.Android.Tools.Bytecode/Log.cs

@@ -15,6 +15,8 @@ public static void Warning (int verbosity, string format, params object[] args)
 			log (TraceLevel.Warning, verbosity, format, args);
 		}
 
+		public static void Warning (int verbosity, string message) => Warning (verbosity, "{0}", message);
+
 		public static void Error (string format, params object[] args)
 		{
 			var log = OnLog;
@@ -23,6 +25,8 @@ public static void Error (string format, params object[] args)
 			log (TraceLevel.Error, 0, format, args);
 		}
 
+		public static void Error (string message) => Error ("{0}", message);
+
 		public static void Message (string format, params object[] args)
 		{
 			var log = OnLog;
@@ -31,13 +35,17 @@ public static void Message (string format, params object[] args)
 			log (TraceLevel.Info, 0, format, args);
 		}
 
+		public static void Message (string message) => Message ("{0}", message);
+
 		public static void Debug (string format, params object[] args)
 		{
 			var log = OnLog;
 			if (log == null)
 				return;
 			log (TraceLevel.Verbose, 0, format, args);
 		}
+
+		public static void Debug (string message) => Debug ("{0}", message);
 	}
 }