add Focus[X]() and .focus() for Iso/ApplyIso

[julien-truffaut]

Fix #1064

Use paren

I could make .focus work but not Focus[X]. The problem is that Focus uses the trick to partially apply type parameter

def apply[S] = new MkFocus[S]

class MkFocus[From] {
    def apply: Iso[From, From] = Iso.id
}

as a result Focus[X] is a MkFocus[X] not an Iso[X, X]. Adding paren to apply fixed the ambiguity.

Moved syntax

I had to move .focus() next to the macro extension .focus(_.foo.bar). When both extension methods where in different classes, it confuses the compiler.

The main issue is for Scala 2, we need define .focus() in the macro module.

[kenbot]

Just do the doco fix and you're good to go

[kenbot]

Lgtm