PointerLabelParam: Return valid (or at least informative) arguments.

PointerLabelParam.to_asm() returns a label by default.

If no label exists, instead return a bank and address.
This will not compile (maybe it ought to), but it
simplifies debugging.
get_ram_label: Setup wram labels if they haven't been already.
get_label_for: Return a label or None.

Don't return an address and call it a label.
Merge pull request #70 from yenatch/master

Straightforward label handling in crystal.py.