No-frills PCG32 random number generator implementation

It implements the PCG32 random number generator (and really only that).

let mut g = pcg32::Pcg32::new(0xff30_6525_39eb_eaa9, 0x315b_fae4_8ade_2146);

assert_eq!(g.gen(), 0xf986_95e1);
assert_eq!(g.gen(), 0x7e39_20e2);

This crate is no_std compatible.