Don't "upgrade" white imps to shadow/iron in Call Imp

Old frequencies at low (0-46) and high (92, near the cap of 100)
spell power:

          low    high
Crimson  20/36  10/36
White    10/36   5/36
Shadow    4/36  14/36
Iron      2/36   7/36

New frequencies at the same power levels:

          low    high
Crimson  20/36  10/36
White    10/36  10/36
Shadow    4/36 ~11/36 (actually 8/27)
Iron      2/36  ~5/36 (actually 4/27)

The breakpoint at 47 power (when shadow/iron imps start to become more
common) is still there.
Fix invisible goblin tossing bugs
Trade Ironheart Preserver defenses for HP

Having better defenses than their pals (or even defenses that
were as good!) meant that the player was encouraged to prioritize
the pals first, generally killing the preserver indirectly
(through the injury bond) without ever bothering to interact with
them. A monster that the player shouldn't bother to attack is a
terrible, terrible design.

More hp means that they can actually soak up the damage the player
deals to their pals, and worse defenses mean that you can do more
HP damage by attacking them than their allies.
Tweak Preservers further

Better Minor Healing, worse armour.
Remove some errant rims

Someday, somehow, we shall reach that golden land wherein we never have to
fix %rim lines ever again.
Fixup stairs in Chokoban
Put !ambrosia on DD autopickup (Arrhythmia)

Since it's very very good for them. (Possibly too good...!?)
Add Shadow Creature traps

Summon a few creatures from the current level whenever the player
or a monster in LOS steps on the trap, roughly as a casting of
Shadow Creatures. The monsters are always hostile to the player.

The number of monsters spawned increases with depth.
Don't draw shock serpent discharges on solid features.

It was causing odd visual glitches.
Fix compilation.

GGGRRRUUUNNNTTT
Properly initialise shock serpent discharge effect tile.

More correctly, tell the beam code to init it for us.
Improve elemental evoker inert text
Display xp evoker discharged sprites again
Remove an obsolete function
Merge branch 'master' into evoker-god

Conflicts:
	crawl-ref/source/enum.h