Allow batform players to memorize spells

Incredible buffs to the "vampire bat" conduct.
Turn unpetrifying tree-form players back to wood.
Turn player flesh into data

Re: de-petrification messages.

Only functional change is that shadow form now turns back to
shadow - though perhaps it shouldn't be petrifiable at all...?
Simplify (PleasingFungus).
Some minor mindexicide.
Use monster::move_to_pos more widely.
Add monster::swap_with, use it in several locations.
Improve Wild Magic card (theTower).

It now causes miscasts on enemies and restores magic for each one affected.
As it is no longer (entirely) bad, it is moved from punishment to wonders.
Also fixes monster malign gateway miscasts from being hostile towards the
player.
Implement and use fair_adjacent_iterator.

Also correcting the spore explosion directional bias introduced in
0.16-a0-2679-gdd6be42.
Simplify some set logic.
Don't pretend a few functions can fail.
Don't reduce crawlie merge chance when full aboms are adjacent.
Move statue and wisp form rPetrify into form data.
Make player shadows insubstantial (AreBrandon)

At least for rPetrify purposes.
Whitespace fix.

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Fix some bracing/indentation
Tweak shard shrikes

Band size from 3-5 to 2-4; base attack damage from 27 to 21.

Throw Icicle is enough of a threat for high-AC characters.
Fix several cases of wonky indentation.

I gave up on changing the weird alignments like:

    const int foo = 1;
          int bar = 0;
Fix braces.

This reminds me that we should standardise on rules for braces in
expressions, particularly multi-line lambdas and initializer lists.
Add Cigotuvi's Embrace

Grab all nearby corpses and turn them into armour + sh. (SH for
consistency with the bone plates mutation, and also to make it a
little more distinctive.)

1/3 of a point of SH & AC per skeleton, 2/3rds per corpse,
minimum one point if you hoovered any corpses/skeletons at all.
casting. Multiple casts stack. Taking damage or blocking hits has
a chance of causing you to lose 1 point; chance decreases with
spellpower. Incompatible with Stoneskin/Ozo's. Does not decay
over time, ironically.

Intent is to replace Twisted Resurrection, which has been
considered a Problem Spell for some time, with another & more
novel use for corpses.