VIP: Change contract keyword to interface

[fubuloubu]

Simple Summary

Use interface as the keyword for defining an interface.

Motivation

VIP #885 added Interfaces to Vyper, but it chose the contract keyword, which in retrospect seems inconsistent with it's intended purpose.

Specification

Just change the keyword. Consider deprecating it first. It would look like:

interface MyInterface:
    def do_stuff(): modifying

Backwards Compatibility

This VIP would not be backwards compatible and would require a small change from users.

Dependencies

Depends on #885, which is already implemented

Copyright

Copyright and related rights waived via CC0

[iamdefinitelyahuman]

I'm in favor of this. Coming from Solidity, having an interface marked as a contract at the top, and then the actual contract having no designation - this always felt confusing to me.

[fubuloubu]

Meeting note: Make a sensible error message if contract is used

[fubuloubu]

@iamdefinitelyahuman Shall we target for 0.2.0?