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/field_in_foreign_class.wren

@@ -0,0 +1,9 @@
+foreign class Foo {
+  bar {
+    // Can't read a field.
+    IO.print(_bar) // expect error
+
+    // Or write one.
+    _bar = "value" // expect error
+  }
+}