Missing function `Easy::into_inner`

I can move (in the sense of transferring ownership) a handler into an Easy (via `Easy::new`), and the Easy into a Multi (via `Multi::add`).

I can later move the Easy out of the Multi (via `Multi::remove`). But, I can't move the handler out of the Easy, I can only get a reference via `get_ref` or `get_mut`. It would be useful to be able to do that. I believe the function would be called `into_inner` by convention, like in `RefCell::into_inner`.

This applies to Easy2 as well. Would you accept a PR for adding the function in both places or is there a reason it doesn't exist?