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/class/missing_class_after_foreign.wren

@@ -0,0 +1,2 @@
+// Missing "class".
+foreign blah A {} // expect error