Missing function `Easy::into_inner`

[jscheid]

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?

[sagebind]

This sounds like a useful addition to me, go for it!