First pass at implementing foreign classes.

Most of the pieces are there:

- You can declare a foreign class.
- It will call your C function to provide an allocator function.
- Whenever a foreign object is created, it calls the allocator.
- Foreign methods can access the foreign bytes of an object.
- Most of the runtime checking is in place for things like subclassing
  foreign classes.

There is still some loose ends to tie up:

- Finalizers are not called.
- Some of the error-handling could be better.
- The GC doesn't track how much memory a marked foreign object uses.

test/language/inheritance/inherit_from_class.wren

@@ -1 +1 @@
-class Subclass is Class {} // expect runtime error: Subclass cannot inherit from Class.
+class Subclass is Class {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Class'.

test/language/inheritance/inherit_from_closure.wren

@@ -5,4 +5,4 @@ var ClosureType
   ClosureType = Fn.new { IO.print(a) }.type
 }
 
-class Subclass is ClosureType {} // expect runtime error: Subclass cannot inherit from Fn.
+class Subclass is ClosureType {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Fn'.

test/language/inheritance/inherit_from_fiber.wren

@@ -1 +1 @@
-class Subclass is Fiber {} // expect runtime error: Subclass cannot inherit from Fiber.
+class Subclass is Fiber {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Fiber'.

test/language/inheritance/inherit_from_fn.wren

@@ -1 +1 @@
-class Subclass is Fn {} // expect runtime error: Subclass cannot inherit from Fn.
+class Subclass is Fn {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Fn'.

test/language/inheritance/inherit_from_list.wren

@@ -1 +1 @@
-class Subclass is List {} // expect runtime error: Subclass cannot inherit from List.
+class Subclass is List {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'List'.

test/language/inheritance/inherit_from_map.wren

@@ -1 +1 @@
-class Subclass is Map {} // expect runtime error: Subclass cannot inherit from Map.
+class Subclass is Map {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Map'.

test/language/inheritance/inherit_from_nonclass.wren

@@ -1 +1 @@
-class Foo is 123 {} // expect runtime error: Must inherit from a class.
+class Foo is 123 {} // expect runtime error: Class 'Foo' cannot inherit from a non-class object.

test/language/inheritance/inherit_from_range.wren

@@ -1 +1 @@
-class Subclass is Range {} // expect runtime error: Subclass cannot inherit from Range.
+class Subclass is Range {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'Range'.

test/language/inheritance/inherit_from_string.wren

@@ -1 +1 @@
-class Subclass is String {} // expect runtime error: Subclass cannot inherit from String.
+class Subclass is String {} // expect runtime error: Class 'Subclass' cannot inherit from built-in class 'String'.

test/language/inheritance/inherit_methods.wren

@@ -22,5 +22,3 @@ bar.method(1, 2) // expect: bar
 bar.method(1, 2, 3) // expect: foo
 bar.method(1, 2, 3, 4) // expect: bar
 bar.override // expect: bar
-
-// TODO: Prevent extending built-in types.