[coop handles] Add handles for interior pointers / valuetype handles

[coop handles] Add handles for interior pointers / valuetype handles

- Call mono_handle_stack_scan for both the precise and the imprecise gc scans.
- For now assert during scan that all handles are to the beginning of an
  object.
- Add TYPED_VALUE_HANDLE_DECL Macro to declare valuetype handles:
     TYPED_VALUE_HANDLE_DECL (MonoMethodInfo);
     /* Declare MonoMethodInfoValueHandle and MonoMethodInfoValueHandleOut */
- Add MONO_HANDLE_NEW_GET_VALPTR (h, type, field) macro to create a new handle that
  points to "field" (of type "type") in the object "h".