Improve filter processing and clean up API

[m-m-adams]

WIP on #105

• Move filterset to a public/private API
• Integrate filterset config into filterset
• Split out individual hp/lp filters into their own classes
• Move stereo rendering into filter classes to remove explicit sample increment calls in render functions

Summary: ~10% gain in mono filtering, 60% gain in stereo filtering, much easier API
No audible changes

[m-m-adams]

Sorry for all the force pushes, I'm bughunting and needed to clean up the commits to be atomic

[m-m-adams]

Initial tasks are all complete now, but I've found a new one that will be necessary

• Move stereo rendering into filter classes to remove explicit sample increment calls in render functions

[stellar-aria]

Just some nitpicks, but at least one (the git conflict) is blocking. Otherwise LGTM.

[sapphire-arches]

couple more minor nitpicks but otherwise LGTM. Nice cleanup, thanks!

[m-m-adams]

Cool - I'll address those and I think I can move the boiler plate linear to log frequency stuff from the filter implementations to the template as well

[m-m-adams]

Should be good to go now