Allow docstrings in interface definition

[SBuercklin]

It'd be convenient to support something like the following syntax for documenting interfaces:

@required Foo begin
    "This is a docstring for bar(::Foo))"
    bar(::Foo)
end

There are ways of working around this, e.g. we can use @doc or just define docstrings as

"This is a docstring for bar(::Foo)"
bar