Use Rayon for data parallelism!

[mominul]

Here we use rayon for parallelism of dictionary searching of phonetic suggestion code. I have done the benchmarks and got 1.20x speedups!

name           old ns/iter  new ns/iter  diff ns/iter   diff %  speedup 
bench_a        456,249      361,669           -94,580  -20.73%   x 1.26 
bench_aro      537,914      461,900           -76,014  -14.13%   x 1.16 
bench_bistari  716,478      598,755          -117,723  -16.43%   x 1.20

For getting better support of rayon, we use the hashbrown's HashMap. But I am not using it in other places because it'll be the default HashMap implementation of the Rust standard library. So we'll get them free in next Rust versions.

[mominul]

cc @Adyel