Vyper interface compilation

[romanagureev]

Simple Summary

Check correctness of .vyi file.

Motivation

There is no way to check if .vyi file alone was written correctly.

Specification

Something like:

 vyper Interface.vyi

[PatrickAlphaC]

Would also be helpful to be able to get the abi from an interface, ie:

vyper Interface.vyi --abi

[charles-cooper]

tentatively tagging for inclusion in 0.4.1

[romanagureev]

Note: discussed with Charles that should compile only with flags -f ast,annotated_ast,interface,external_interface

[charles-cooper]

And abi!