basic_filters: optimize RC highpass & bandpass filters

For both RC12 and RC24 filter types: handle lowpass separately, because we can then calculate highpass & bandpass with less operations.
This shouldn't affect the performance of lowpass, but probably will make highpass and bandpass a bit faster.

Addendum: some further optimizations added after IRC conversations with softrabbit, including some optimizations for the formant filter, and some optimizations to coeff calculations, which should help with modulated filters.