Support double precision

[jd-13]

This issue is to track double precision support across plugins:
jd-13/Richter-Tremolo#6
jd-13/MONSTR-Stereo-Imaging#13
jd-13/Songbird-Formant-Filter#5
jd-13/Carve-Distortion#15

Currently it appears only VST2/VST3 supports double precision, this doesn't seem to be a particularly important feature right now.

https://forum.juce.com/t/audio-plugins-64-bit-processing-double-precision-dsp-in-audiounits/28173/5