Clean up post-split main.asm. Get rid of static wram addresses. Lay out a foundation for Yellow.

- More splits (home.asm,  etc).
- Use labels instead of static values for all wram addresses. This allows wram to be modified freely.
- Add the TX_BCD macro.
- Use structs in wram.
- Start provisionally building yellow.
- Use a macro for predefs instead of awkward predef constants.
- Rename a bunch of music commands.