libcore: rand: Use a pure Rust implementation of ISAAC RNG

This replaces the wrapper around the runtime RNG with a pure Rust implementation of the same algorithm. This is much faster (up to 5x), and is hopefully safer.

There is still (a little) room for optimisation: testing by summing 100,000,000 random `u32`s indicates this is about ~~40-50%~~ 10% slower than the pure C implementation (running as standalone executable, not in the runtime).

(Only 6d50d55 is part of this PR, the first two are from #6058, but are required for the rt rng to be correct to compare against in the tests.)