memory.c, others: Add #define GAMEPAK_FITS_IN_RAM, which is for platforms on which the entire largest Game Pak can be loaded without needing to load on demand.

GCW Zero: Build with GAMEPAK_FITS_IN_RAM.
common,h, memory.c: Fix gcc's warnings about type-punned pointer aliasing.
cpu_asm.c, mips/emit.h: Fix gcc's "Suggest parentheses around '-' in operand of '&'" warnings.
DSTwo: Get rid of 3 compiler warnings.
Developer documentation: Commit a file explaining all the conditional compilation options available in ReGBA.