Support externally-hashed keys

[vlovich]

I have a use-case where:

1. I want to use a different hash function (specifically xxh3 which is typically faster than wyhash)
2. I have multiple hash functions to lookup the same key. I can thus amortize so that the hashing only happens once outside of Mphf.

This provides savings so that the lookup within the hash function is basically the same cost as a u64 regardless of the true size of the key input. Obviously the hash still has to be done somewhere, so it's not totally free but as mentioned the savings grow the more hash functions I need to interrogate and it lets me use a faster xxh3 hash.

I tried to keep the identical hash results if you use anything other than the new ExternallyHashed type while simultaneously carving out a fast path for it to shine (I've also in theory fast-pathed construction/lookups for integer keys where we can skip constructing the WyHash state object but I didn't bother benchmarking the actual improvement there).

[pmarks]

Interesting... I'm trying to figure out if we could avoid switching from Hash to SeedableHash as the trait required for keys. This feel quite onerous to users of the library to have to implement a new trait, rather than using Hash which is much more likely to be implemented for some type one might want to use as a key. Hash is very convenient to derive, whereas SeedableHash would need to be implemented by hand, and would run into orphan issues for a type the user doesn't control. That feels like a big blocker to me.