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.
basic_filters: further optimization
Use qBound in formant, and return early from calcFilterCoeffs so we don't do needless calculations
basic_filters - even more optimization
Use more efficient interpolation, optimize some calculations
basic_filters: replace div with mul
Merge pull request #895 from diizy/filters

basic_filters: optimize RC highpass & bandpass filters