sdk: add support for receiver data and framework.New

[vancluever]

This adds support for:

• Receiver data in sdk.GetReq, and sdk.GetResponse, in the form of the "Context" field. This will be passed from the runtime in the event of method calls on data returned from an import during earlier calls. "Callable" in the response also indicates to the runtime whether or not a return value can receive methods in future call.

• The introduction of the framework.New interface. When present on an import root, this will allow for instantiation of any namespace that the import author wants, depending on the structure of the data passed along in the receiver.

• The appropriate handling for framework.New, and the protocol modifications for the new fields.

[vancluever]

PR depends on #23. The protocol stuff is still incoming, but what is in here should give a great idea on how this will work on the plugin implementation side.

[vancluever]

All work has been completed and it is now ready for review.