taproot swaps

[JssDWt]

This PR adds taproot swap support.

Changes in a nutshell:

• the term 'redeem' was renamed to 'claim'
• the public api has changed
• swaps are now taproot swaps, with
  • a claim script spend path
  • a refund script spend path
  • a cooperative musig key spend path
• there's a cooperative refund added
• lock time is now absolute. The default locktime is 1008 blocks from when the swap address was made.

Note about the musig2 implementation:

There is no musig2 implementation in rust-bitcoin at the moment. There is a PR in rust-secp256k1 though. Combining the version from that PR with the taproot parts of rust-bitcoin goes well, as long as keys are serialized/deserialized to convert the incompatible structs from one package to another.

review notes

Apologies for the huge PR. For review I think the best option is:

• check the public server interface in swap.proto
• check the pay and refund logic in public_server.rs
• check the taproot logic in swap_service.rs