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/api/returns.c

@@ -27,7 +27,7 @@ static void returnFalse(WrenVM* vm)
   wrenReturnBool(vm, false);
 }
 
-WrenForeignMethodFn returnsBindForeign(const char* signature)
+WrenForeignMethodFn returnsBindMethod(const char* signature)
 {
   if (strcmp(signature, "static Api.implicitNull") == 0) return implicitNull;
   if (strcmp(signature, "static Api.returnInt") == 0) return returnInt;