unnecessary transmutes

[Lokathor]

• https://github.com/rust-random/rand/blob/master/rand_core/src/impls.rs
• https://github.com/rust-random/rand/blob/master/rand_pcg/src/pcg128.rs

Not sure when this code was written, but by replacing rng.next_u64().to_le() with rng.next_u64().to_le_bytes() we can cut down on some transmutes.

Now, yes, of course to_le_bytes does the transmute internally, but we should avoid unsafe and let the standard library do it as often as possible simply on principle.

That method was stable in 1.32, and the README badge indicates that that's the minimum rust version of rand these days, so it should be an easy change.

• https://github.com/rust-random/rand/blob/master/src/distributions/float.rs#L98

Has an old TODO for when the compiler gets to 1.25, which we're at.

[vks]

This was fixed with #840.