Remove bounds checks from filter_signal

[valadaptive]

The gammatone filterbank is the hottest function in this code by far.

I noticed that the filter_signal function seemed to contain a lot of bounds checks, one per coefficient. Because bounds checks are control flow, they can prevent many useful optimizations.

By passing numerator_coeffs, denom_coeffs, and init_conditions as references to fixed-size arrays rather than slices, the compiler knows all our accesses are inbounds, and can omit the bounds checks.

This allows the code to be vectorized, and is 30-40% faster on my benchmarks.

Performance can probably be improved a lot further with some heavier reworks, but this is a small change that's easy to review.