cache_hash: hash function change
cache_hash: hash function change

This makes the `cache_hash` hash function easier to replace.

BLAKE3 would be a natural fit for hashing large files, but:
- second preimage resistance is not necessary for the cache_hash use cases
- our BLAKE3 implementation is currently very slow

Switch to SipHash128, which gives us an immediate speed boost.

raulgrell/zig,zig-lang/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig