Update the equals function to fall back to structural equality

When comparing two values that are:

1. of the same type; and
2. do not provide a 'fantasy-land/equals'-method; but
3. also don't have a known type with a Z-provided implementation

then previously Z.equals would return false, but this false is just as
reliable as a true would be.

After this commit, instead of returning false, the Z.equals now returns
the structural equality of the two inputs.

index.js

@@ -1178,12 +1178,16 @@
 
   //# equals :: (a, b) -> Boolean
   //.
-  //. Returns `true` if its arguments are of the same type and equal according
-  //. to the type's [`fantasy-land/equals`][] method; `false` otherwise.
+  //. Returns `true` if the two arguments given are structurally equal. When
+  //. comparing two values, equality is determined based on:
   //.
-  //. `fantasy-land/equals` implementations are provided for the following
-  //. built-in types: Null, Undefined, Boolean, Number, Date, RegExp, String,
-  //. Array, Arguments, Error, Object, and Function.
+  //. - The types of the two values. Values of differing types unequal.
+  //. - The [`fantasy-land/equals`][] method of the first value when present.
+  //. - For Null, Undefined, Boolean, Number, Date, RegExp, String, Array,
+  //.   Arguments, Error, Object, and Function, Sanctuary Type Classes provides
+  //.   `fantasy-land/equals` implementations.
+  //. - Any other types are structurally compared by recursively comparing the
+  //.   values at each of the keys.
   //.
   //. The algorithm supports circular data structures. Two arrays are equal
   //. if they have the same index paths and for each path have equal values.
@@ -1219,9 +1223,9 @@
 
       $pairs.push ([x, y]);
       try {
-        return Setoid.test (x) &&
-               Setoid.test (y) &&
-               Setoid.methods.equals (x) (y);
+        return Setoid.test (x) ?
+               Setoid.methods.equals (x) (y) :
+               Object$prototype$equals.call (x, y);
       } finally {
         $pairs.pop ();
       }

test/index.js

@@ -675,7 +675,7 @@ test ('equals', function() {
   eq (Z.equals (Math.sin, Math.cos), false);
   eq (Z.equals (Identity (Identity (Identity (0))), Identity (Identity (Identity (0)))), true);
   eq (Z.equals (Identity (Identity (Identity (0))), Identity (Identity (Identity (1)))), false);
-  eq (Z.equals (Useless, Useless), false);
+  eq (Z.equals (Useless, Useless), true);
   eq (Z.equals (Array.prototype, Array.prototype), true);
   eq (Z.equals (Nothing.constructor, Maybe), true);
   eq (Z.equals ((Just (0)).constructor, Maybe), true);