Allow docstrings in interface definition

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
```