Allow __default__ function in interfaces

[pcaversaccio]

Summary

The Vyper compiler currently disallows defining the fallback function __default__() as part of an interface definition. E.g. the following contract will raise with:

# pragma version ~=0.4.0
interface FooBar:
    def __default__() -> uint256: payable

implements: FooBar

@external
@payable
def __default__() -> uint256:
    return msg.value

contracts/Precompile.vy
FunctionDeclarationException:Default functions cannot appear in interfaces

  contract "contracts/Precompile.vy line 3:4 
       2 interface FooBar:
  ---> 3     def __default__() -> uint256: payable
  -----------^
       4

This can become problematic, if you want to enforce custom interface definitions (e.g. my snekmate timelock_controller could be transformed into an interface, which would require __default__() to be complete). There might also be some EIPs or future EIPs that require this. We should discuss here if we should lift this restriction or not.