Fix asymptotic hashing performance

[abonander]

The previous implementation took a single 64-bit hash and split it into 3 21-bit components.

This implementation takes 2 64-bit hashes and splits them into 3 32-bit values, discarding the upper 32 bits on the second hash. For comparison

• at abonander@ecb9fd5 (before hashing changes):

$ time cargo run --bin gen_hash_test --release --features criterion
   Compiling phf_shared v0.7.24 (/home/austin/rust/rust-phf/phf_shared)
   Compiling phf_generator v0.7.24 (/home/austin/rust/rust-phf/phf_generator)
    Finished release [optimized] target(s) in 0.70s
     Running `/home/austin/rust/rust-phf/target/release/gen_hash_test`
112.18user 0.11system 1:51.61elapsed 100%CPU (0avgtext+0avgdata 262776maxresident)k
0inputs+6280outputs (0major+58275minor)pagefaults 0swaps

• at abonander@ab595fe (after hashing changes):

$ time cargo run --bin gen_hash_test --release --features criterion
   Compiling phf_shared v0.7.24 (/home/austin/rust/rust-phf/phf_shared)
   Compiling phf_generator v0.7.24 (/home/austin/rust/rust-phf/phf_generator)
    Finished release [optimized] target(s) in 0.77s
     Running `/home/austin/rust/rust-phf/target/release/gen_hash_test`
4.83user 0.11system 0:04.14elapsed 119%CPU (0avgtext+0avgdata 265640maxresident)k
0inputs+6168outputs (0major+58545minor)pagefaults 0swaps

I didn't use the test data from #132 as I didn't want to include such a large file in the repo; still, this seems to demonstrate a significant reduction in asymptotic behavior with the algorithm.

[abonander]

cc @sfackler @derekdreery

[sfackler]

Also worth trying the 128-bit output in siphasher itself - might be faster than hashing twice: https://docs.rs/siphasher/0.3.0/siphasher/sip128/index.html

[sfackler]

Really interesting that this has such a huge impact!

[abonander]

Also worth trying the 128-bit output in siphasher itself - might be faster than hashing twice: https://docs.rs/siphasher/0.3.0/siphasher/sip128/index.html

It's faster by maybe 5% but it simplifies the code a bit so an overall positive change.

[abonander]

Failure on nightly is due to a bug that has been fixed; build should pass if rerun tomorrow. rust-lang/rust#62562

[abonander]

Passed, merging.