Refactor potion.cc to be a map

...from potion enum to potion effects, in a roughly similar structure to
transform.cc, etc.

Adjust other files acordingly; banish many inappropriate uses of potion_effect
when not actually drinking potions.

[Committer's note: fixed some things, rewrote stuff, etc]
Improve ambrosia messenging

Always print the ending message, regardless of whether it ended
by confusion ending or simple timeout.
Deprecate POT_SLOWING

No longer used anywhere, huzzah!