a738754f67 Add TxIdentifier trait (Tobin C. Harding)
Pull request description:
Add a new trait `TxIdentifier` that abstracts over the `Txid` and `Wtxid` types. We make `AsRef` a super trait so that the new trait needs no methods.
Seal the trait so consumers of the library cannot implement it.
Use the new trait in:
- the `bip152` module to tighten up the `with_siphash_keys` function
- as a trait bound on the `Leaf` associated type in the `MerkleNode` trait
ACKs for top commit:
apoelstra:
ACK a738754f67
Kixunil:
ACK a738754f67
Tree-SHA512: a7bda26a4a5107f96b24ea3c163286a7ab21a817bdec3434b3ab27d78e99c0548a7362a2271d362b89038c80d9251767c0d62e1df702ef57d9edaf141ab55cd6