[Bug #20253]

All the way down to Ruby 1.9, `Proc`, `Method`, `UnboundMethod`
and `Binding` always had their own specific clone and dup routine.

This caused various discrepancies with how other objects behave
on `dup` and `clone. [Bug #20250], [Bug #20253].

This commit get rid of `CLONESETUP` and use the the same codepath
as all other types, so ensure consistency.

NB: It's still not accepting the `freeze` keyword argument on `clone`.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>