Don't unnecessarily borrow &self as mutable

[valadaptive]

This might be a breaking change under semver rules--I don't know enough to be confident.

The get_noise functions don't actually need to borrow self mutably, and doing so can be somewhat limiting. For instance, I want to generate noise in parallel on multiple threads. That cannot be done with &mut self, as Rust treats that as an indication that that function mutates some shared state within the generator, which cannot be done safely on multiple threads.

However, since these functions don't actually mutate any internal state, they're perfectly safe to call from multiple threads at once--Rust just doesn't know it. That's what this PR fixes.

[Keavon]

Ah, yes, good fix! I don't think this would be a breaking change because existing callers, I'm pretty sure, shouldn't have any trouble using their existing code.