Let actor::hurt accept a kill_method_type, source, and aux.

...which it then passes along to ouch() if it's the player.

Lets us merge some player and monster handling and fix some death
messages (Freeze, shock serpent discharges, ...)
KILLED_BY_COLLISION, for collision death messages.

Collided with an orc (7 damage) ... caused by a deep elf mage

This is deliberately separate from the previous commit in case we only
want to land the former.
Give singularities' direct damage a proper death message.

Killed by gravitational forces
... invoked by a singularity
... created by an ancient lich
 (23 damage)

or,

blasted by a singularity (gravitational forces) (created by an ancient
lich)

Possibly the "invoked by" could be changed somehow, but this suffices.