Restore these methods for StringIO

StringIO currently still calls one or both of these cat19 methods,
and will need time to migrate away from them. We may not be able
to safely remove these after one major release cycle, so I am
reverting this deletion with extra notes for future evaluation.

The migration of StringIO will be tracked in ruby/stringio#83
and we will be backporting the new names to try to fast-track the
migration in the next year.

core/src/main/java/org/jruby/RubyString.java

@@ -1374,6 +1374,13 @@ public final RubyString cat(byte[] str, int beg, int len) {
         return this;
     }
 
+    // Needs to remain in place until StringIO has migrated to the new methods
+    // See https://github.com/ruby/stringio/issues/83
+    @Deprecated
+    public final RubyString cat19(RubyString str2) {
+        return catWithCodeRange(str2);
+    }
+
     // // rb_str_buf_append against VALUE
     public final RubyString catWithCodeRange(RubyString str2) {
         ByteList other = str2.getByteList();
@@ -1389,6 +1396,13 @@ public final RubyString cat(ByteList other, int codeRange) {
         return this;
     }
 
+    // Needs to remain in place until StringIO has migrated to the new methods
+    // See https://github.com/ruby/stringio/issues/83
+    @Deprecated
+    public final int cat19(ByteList other, int codeRange) {
+        return catWithCodeRange(other, codeRange);
+    }
+
     // rb_str_buf_append against ptr
     public final int catWithCodeRange(ByteList other, int codeRange) {
         return EncodingUtils.encCrStrBufCat(metaClass.runtime, this, other, other.getEncoding(), codeRange);