Change mutability in `push` and `try_pop`
Change mutability in `push` and `try_pop`

This makes it possible to use `chase_lev` as a drop-in replacement for
the deque in the `deque` crate, especially in `rayon`

crossbeam-rs/crossbeam,crossbeam-rs/crossbeam,aturon/crossbeam,crossbeam-rs/crossbeam,crossbeam-rs/crossbeam