Use clamp_s16 where appropriate.
Remove unneeded state variables.
Fix top comment
Move MP3 prototype into its own header.
Move acmd parsing to alist_naudio
Regroup all ucode headers into a single header.
Rename alist_internal.h into alist.h
Refactor memory functions.
Refactor simple alist buffer accesses.
Implement NAUDIO_02B0.

To implement NAUDIO_02B0, we emulate instructions located at 0x12b0 (inside SETVOL), which
overwrite lowest part of right rate.
Indeed, despite its address 0x02b0 (ie DMEM), SP can only execute in IMEM.
Merge pull request #19 from bsmiles32/refactorings

Refactorings
Merge pull request #20 from bsmiles32/naudio_02b0

Implement NAUDIO_02B0.