These files change infrequently, and not much header shuffling is required.
We don't add everything in src/util/ yet, because IWYU makes some
dubious suggestions, which I'm going to follow up with upstream.
Move the hashers that we use for hash tables to a common place.
Moved hashers:
- SaltedTxidHasher
- SaltedOutpointHasher
- FilterHeaderHasher
- SignatureCacheHasher
- BlockHasher