Currently, both the MPB and OSB code contain duplicate code and definitions for AICA related stuff.

Notably, the pan related calculations are duplicated, and the AICA channel register layout and serialisation/deserialisation is too.

Ideally, all of what's in common.hpp should be moved to aica.hpp, and the channel registers with their serialisation code should too (i.e., everything up until FilterEnvelope filter). This could make the code look more verbose, but could be beneficial.