Use `bitcoin_hashes` rather than `ring` for hashing

Use `bitcoin_hashes` rather than `ring` for hashing

While `ring` is great, it struggles with platform support and has a
fairly involved dependency tree due to its reliance on C backends.

Further, while the `RustCrypto` org tries to stick to Rust, in
doing so it takes on more (unnecessary) dependencies and has a
particularly unusable MSRV policy. Finally, its contributor base
has historically not been particularly friendly.

Thus, for the best platform support, we'd like to avoid both. Here
we take the first of several steps towards that goal, using
`bitcoin_hashes` for our SHA-1/SHA-2 operations instead.