deps: update everything

This adds one new dependency, maybe-uninit, which is brought in by
crossbeam-channel[1]. This is to apparently fix some unsound code
without bumping the MSRV. Since ripgrep uses the latest stable release
of Rust, the maybe-uninit crate should compile down to nothing and just
re-export std's `MaybeUninit` type.

[1] - crossbeam-rs/crossbeam#458