Restore save state functionality

[jamesathey]

Somewhere in the evolution of Nes_Snd_Emu into its inclusion in Game_Music_Emu, the save state functionality was lost. The apu_snapshot code from 0.1.7 does not fit at all with the current conventions.

VRC6 has an implementation of save_state(), as does Blip_Buffer, but the other expansion audio chips as well as the main APU do not have implementations.

Using the old apu_snapshot code as a reference and the VRC6 code as a current example, implement the save_state() function for the 2A03, followed by the remaining expansion audio chips.