Interface formatting

[iamdefinitelyahuman]

blackadder currently handles interfaces as though they are class methods:

• the mutability is placed on a new line below the function selector
• a blank line is left between each function definition

interface Foo:
    def bar(a: uint256, b: uint256):
        view

    def baz(a: address):
        nonpayable

I think in this case, a more succinct format is actually more readable. Each function definition should be placed on a single line where possible, and there should not be a blank line between functions:

interface Foo:
    def bar(a: uint256, b: uint256): view
    def baz(a: address): nonpayable

[spinoch]

It probably also makes sense to implement #7 before we work on this