Split up a bad ASSERT (wheals)
Use a #define'd key
Clean up monster corruption

Abomination malmutation is broken (doesn't actually make visual
changes, like it seems to think it does), but that was broken
before too. Similarly, monster malmut & corrupt should be
differentiated, but that's also not a problem introduced by this,
so...
Actually use TILE_NUM_KEY (oops)
Fix abomination "mutation"

They were marked undead, so couldn't be mutated.

Also arguably cleanup some chaos spawn code.
Oops
Spellify Wretched Stars' Corrupting Pulse
Move functions only used elsewhere out of mon-abil.cc.

Some into the new mon-tentacle.cc!
Clean up _ms_waste_of_time.
Centralise monster spell safety checks in _ms_waste_of_time.

The default diff for this commit is really awful; --patience makes
it much more clear what is happening.
Fix some monster tracers.

Englaciation can increase slow duration, you can always see invisible
allies, and undead have been able to cast Regeneration forever.
Don't abort out of all monster spellcasting.

WWWHHHEEEAAALLLSSS
Make the shadow monster's invisibility a spell.
Spellify Snorg's berserk.

Includes the odd full HP special case for posterity.
Spellify siren songs.

And avatar songs, of course.