Add benchmarks
Add benchmarks

This adds benchmarks to compare phf with btreemaps, hashmaps,
and string matches.

sfackler/rust-phf,erickt/rust-phf,djudd/rust-phf,dinfuehr/rust-phf,Bobo1239/rust-phf,dinfuehr/rust-phf,brson/rust-phf,djudd/rust-phf,erickt/rust-phf,brson/rust-phf,sfackler/rust-phf,Bobo1239/rust-phf