First phase of a Hash reimplementation

This PR merges `IterBytes` and `Hash` into a trait that allows for generic non-stream-based hashing. It makes use of @eddyb's default type parameter support in order to have a similar usage to the old `Hash` framework.

Fixes #8038.

Todo:

- [x] Better documentation
- [ ] Benchmark
- [ ] Parameterize `HashMap` on a `Hasher`.