Rewrite DSP plugin interface.
Rename rarch_dsp.h.
Remove broken DSP plugs.

They will all have to be properly rewritten for new interface.
Add new DSP plug "Panning".
Some Panning fixes.
Remove RGUI_SETTINGS_BIND_DSP_CONFIG.
Add builtin crap for DSP plugs.
Rewrite IIR DSP filter.
Add the IIR filter types.
Add Echo DSP filter.
Use defaults for Echo.
Merge pull request #694 from libretro/dsp-rewrite

DSP rewrite