Switch to `chacha20-poly1305` crate

[TheBlueMatt]

rust-bitcoin now has a crate for chacha20-poly1305. We should try to use it and see if the API suffices for all our weird stuff. https://docs.rs/chacha20-poly1305/latest/chacha20_poly1305/

[tnull]

For reference, over at vss-client we successfully made the switch a while back: lightningdevkit/vss-client#42