Migrate to `hybrid-array`

[rozbb]

generic-array is getting bigger and harder to audit. We also don't use almost any of the features. It'd be nice to move to the smaller (and more future-oriented) hybrid-array crate. This should a straightforward move, once digest v0.12 (or v0.11?) and aead v0.6 drops, using hybrid-array.

Patching is really annoying right now so I'll just wait until these are out before I make a PR.