Enable Macros on interfaces

[Jooseppi12]

[<Macro(typeof<...>)>]
type X =
    abstract Y: int -> int
    abstract Z: string -> string

This combined with #1383 allows us to add more metaprogramming capability

[Jand42]

@Jooseppi12 What happens when the interface is implemented? Should it be allowed?
Also, if not for #1383, above example makes no sense if the interface is not a proxy.