[remoting] Coop handles for proxy objects (#4208)

[remoting] Coop handles for proxy objects (#4208)

* [remoting] Use coop handles in several functions

  - icall System.Runtime.Remoting.Proxies.RealProxy.InternalGetTransparentProxy
  - mono_remote_class
  - mono_remote_class_vtable
  - mono_upgrade_remote_class
  - mono_object_handle_get_virtual_method
  - mono_object_handle_isinst
  - mono_object_handle_isinst_mbyref

* [reflection] Use handles in System.RuntimeTypeHandle.IsInstanceOfType

* [runtime] Rewrite mono_object_castclass_mbyref
  in terms of mono_object_handle_isinst_mbyref.

  This is an external API function that isn't used inside the runtime.

* [runtime] Remove mono_object_isinst_mbyref_checked

  It was a runtime internal funciton.

  All of its former callers have been converted to use mono_object_handle_isinst_mbyref.