[META] Re-invigorating the compiler directive discussion

These directives (including current os thingy) don't have # in front of
them. I think there is inconsistency here.

Consider:

#if 2 + 3 == 5
//...
#end

let v: Bool = #os(iOS)

#if #os(iOS)
//...
#end

I mean, #os and its suggested look-a-likes are not normal functions and so
should have # in front of them. #if has orthogonal responsibilities.

- Anton