Bodyswap cleanup

1. `$swappingSlave` is now an ID instead of a cloned object.
2. Eliminate many direct slave index lookups.
3. Clearly comment that `origBodyOwner` and `origBodyOwnerID` actually store exactly opposite data despite having pretty much the same name.
4. Fix various bugs dealing with tracking original bodies.
5. Fix body swap reactions upon regaining one's original body, and don't emit the same spuriously.