[coop] Change handles to be pointers (#3249)

[coop] Change handles to be pointers (#3249)

Previously handles were pointer-sized structs.  Unfortunately not all
calling conventions treat pointer-sized structs as pointers, so if we
wanted to interact with handles from wrappers, we would have to do
something complicated.

With the new design, `TYPED_HANDLE_DECL(MonoFoo)` will expand to:
```c
    typedef struct {
        MonoFoo *__obj;
    } MonoFooHandlePayload;
    typedef MonoFooHandlePayload *MonoFooHandle;
```