Trace call sites in disassembler

[jeremyBanks]

• Update RomByte structure to add a Vec of call sites for each call target

This is a nice idea but right now we'd have no way to represent this information in the disassembled form we go through to get human-readable representation.

If we even added labels to our disassembled code blocks, we could add this information in there.

If a block is only ever jumped to from itself, maybe we call it a "loop"?