No-frills PCG32 random number generator implementation

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

```rust let mut g = pcg32::Pcg32::new(0xff30652539ebeaa9, 0x315bfae48ade2146);

asserteq!(g.gen(), 0xf98695e1); asserteq!(g.gen(), 0x7e3920e2); ```

This crate is no_std compatible.