Implementations - Proxies - Rust + Mx

[virgil-serbanuta]

Needed for #34

Needs:

• Object returned by a #[proxy] method (Proxy). This should allow calls to the proxy's methods
• Object returned by one of the method calls above (CalledProxy maybe). This should allow either transfers (esdt(tuple)) or execute_on_dest_context().
• Object returned by a transfer (maybe still a CalledProxy object, but with some fields filled in). Should allow execute_on_dest_context() calls
• execute_on_dest_context() should call a hook for executing another contract, and, perhaps, return a result.