Rework trap generation

Instead of mostly generating mechanical traps & then squashing
them (except in Slime, which had more than ten times as many
traps as the rest of the game since it never generated mechanical
traps to begin with), just generate magical traps. Should make
traps actually appear in the game again outside Zot, which I think
is good for variety & excitement.

Trap numbers, ratios, etc have been adjusted.
Examples:
D:1:
	Old: 0.4 Tele, 0.35 Shaft
	New: 1.3 Tele, 0.7 Shaft
D:15/Lair:5/S:1:
	Old: 0.08 Zot, 0.05 Alarm, 0.39 Tele, 0.34 Shaft
	New: 0.07 Zot, 0.75 Alarm, 1.5 Tele, 0.75 Shaft
Zot:1
	Old: 2.01 Zot, 0.05 Alarm, 0.39 Tele, 0.34 Shaft
	New: 2.33 Zot, 0.42 Alarm, 0.85 Tele, 0.42 Shaft
This is partially as an experiment for compensating for monsters
setting off one-use tele traps before you ever see them.
Fix trap generation (wheals, |amethyst)

The other approach would cause early termination.
Turn feat_is_trap() into a feature flag
Don't bounds error on throwing some non-missiles (#9344)

Another crash with more stack trace is at:
    https://dobrazupa.org/morgue/nago/crash-nago-20141225-215909.txt
Re-replace Maxwell's brand

Resistance -> rCorr+rElec, roughly paralleling its old brand.

{resistance} was essentially a placeholder after the removal of
{preservation}.
Don't use Slime walls in Sewers

Not something we want people to get confused about!
Avoid some closets in bailey_axe_4.

Make sure one of the three closets is connected to the rest of the level,
and put exits in the other two.
Spell for the Commonwealth.

<PleasingFungus> Aeon might at least sound cooler...?
Replace a switch with an array lookup
Fix an abyss test.

abyss_short_wait is supposed to not have monsters, and fails if any do
appear, with "A foo is nearby! xN Infinite lua loop detected, aborting."

For one, explicitly dismiss monsters on entering the abyss (rather than
just disabling spawns).  Also, just in case (for example, if we generate
a new abyss area that contains a vault), wait with . rather than 5 so we
will advance the turn counter regardless.
Assert that we don't infinitely recurse on bad unicode.
Hush a warning.

I *hope* we don't get level 2e9 spells.