Updating rust-bitcoin and supporting musig roadmap

We have a goal to update rust-elements to the latest 1.0 versions of rust-bitcoin crates; relatedly but separately we want to cleanup our APIs, and critically, we want to support musig2.

* [ ] rewrite rust-secp256k1-zkp as a wholly independent library which depends on the latest libsecp256k1-zkp
* [ ] port the rust-secp musig2 PR to rust-secp-zkp, merge it, and release it
* [ ] update rust-elements to use that
* [ ] update rust-elements to use `bitcoin-hashes 1.0`
* [ ] update rust-elements to use `bitcoin-consensus-encoding 1.0` rather than own encoding library) (blocked on https://github.com/rust-bitcoin/rust-bitcoin/pull/6184 I think)
* [ ] update rust-elemnets to use `bitcoin-units 1.0` (not yet released)