Rollup merge of #137095 - saethlin:use-hash64-for-hashes, r=workingjubilee

Replace some u64 hashes with Hash64

I introduced the Hash64 and Hash128 types in rust-lang/rust#110083, essentially as a mechanism to prevent hashes from landing in our leb128 encoding paths. If you just have a u64 or u128 field in a struct then derive Encodable/Decodable, that number gets leb128 encoding. So if you need to store a hash or some other value which behaves very close to a hash, don't store it as a u64.

This reverts part of rust-lang/rust#117603, which turned an encoded Hash64 into a u64.

Based on rust-lang/rust#110083, I don't expect this to be perf-sensitive on its own, though I expect that it may help stabilize some of the small rmeta size fluctuations we currently see in perf reports.

Author: matthiaskrgr