Object#identity

I know this seems like a stupid addition, but every time I have something to use something like `{ |foo| foo }` I wish for a method (isn't `(&:identity)` much nicer?). 

I know the name is a bit long, but the more common `id` is blocked by the deprecated `Object#id`. I toyed with `it` for a bit (`collection.group_by(&:it)`) but didn't find that entirely convincing either. Defining `Object#self` works, but I thought that might lead to confusion...