Updating rust-bitcoin and supporting musig roadmap

[apoelstra]

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 0.32.x: Add consensus_encoding to types present in units 1.0 rust-bitcoin/rust-bitcoin#6184 I think)
• update rust-elemnets to use bitcoin-units 1.0 (not yet released)