Include interpreter identity in enclave registration

[masomel]

Given that we support different types of contract interpreters, and plan to add support for AoT WASM contracts, we should have a way to bind an enclave to a specific interpreter identity, and ensure that contracts can only be loaded into enclaves that match the interpreter the contract was built for.

Recommended changes:

• enclave registration should include a field indicating the interpreter type an enclave will run
• contract registration should indicate the interpreter a contract is compiled for
• add_enclave should ensure that a contract's interpreter matches the enclave's interpreter

Initial discussion around this: #259 (comment)
This issue also seems related to #255

[cmickeyb]

This is a nice to have, but can already be done (already IS done) through the info call when initially establishing a connection to an enclave. Suggest that this is ultimately low priority.