For unknown reasons, the text pointer list for Oak's Lab is
duplicated. This is currently implemented by treating the
two text pointer lists as one large list. This leads to
duplicate labels and some confusing logic.

Refactor the code to treat the two text pointer lists
separately. This lets us remove several labels. It also
makes the code which sets the text pointer list
self-documenting.

Use OPP_ constants separate from trainer constants since
valid opponents can be all Pokemon and trainers

and other clean up