Use a fixed RNG
Use a fixed RNG

This will make compiles more consistent and a tiny bit faster since
we're using an XorShiftRng.

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