core: a pure Rust implementation of the ISAAC RNG.
core: a pure Rust implementation of the ISAAC RNG.

This replaces the wrapper around the runtime RNG with a pure Rust
implementation of the same algorithm. This is faster (up to 5x), and
is hopefully safer.

There is still much room for optimisation: testing by summing 100,000,000
random `u32`s indicates this is about 40-50% slower than the pure C
implementation (running as standalone executable, not in the runtime).

rust-lang/rand