Pull in a large number of UNUSED potion tiles/colours

Seemed a pity to let them go to waste.
Remove UNUSED potions (hangedman)
Add a placeholder Hydra Form icon
Add doxygen comments for weapon_def.
CAN-SPAM (zin recite debug messages)
Allow Gozag wrath to affect more than one item per stack (potatolizard)

Introduced in 0.15-a0-1882-g4eab053.
Fix butterfly rims (Bloax)
factor breath definitions out

this should make it more introspectable for monster
Constify and comment.
Externalise that const
Add a new randart name (Lasty, elliptic).
Turn iron devils into rust devils

Less damage, but a touch attack that does nearly-guaranteed
corrosion on hit+damage.
Give demigods have double-stat-ups

On every third level, mainly for the elegance of the design.

This leaves their total stats unchanged, but delays them very
slightly & gives slightly less fine control.
Give innate sustain abilities to demigods (Brannock)

Suits their mechanical theme ("good stats"), and is a nice tiny
buff to match the last commit's tiny nerf. (Not that one was
needed, of course, but there's a symmetry there.)
Move the Serpent of Hell up a tier in pitsprint

Now that it's a threat.
Allow rCorr to generate on randart armour

This increases the distribution of the resistance somewhat since
there's no more preservation and acid damage is more common now. There
was some objection to it being found on weapons and rings, so we'll
try armour only for the time being.
Fix armour randart tiles (|amethyst)

Most of the helmet/robe randart tiles weren't being used,
because item.rnd was never being set. Either of these changes
would have fixed the problem; made both in an attempt to mitigate
future errors, though there's only so much you can do with a
function like that...