Merge fire-based erosion to common codepaths.

Merge fire-based erosion to common codepaths.

This finally eliminates all direct increases of `oeroded` and `oeroded2`
and moves them all to go via `erode_obj()`. They are still manipulated
directly in a few places, but not to erode objects.

This now merges the `fire_damage()` function to a common codepath, used
for items on lava and burning oil, but fire needs more work. There is
still a duplication between `destroy_item()` and `fire_damage()`; the
two codepaths should eventually be merged in some manner so that there
is only one codepath to say "an object was affected by fire". This path
might require some parameters, such as whether the fire will just erode
objects or burn them outright, but that can happen another day.