forgetting spells due to amnesia (trunk only)

forgetting spells due to amnesia (trunk only)

     From the newsgroup, losing spells to amnesia always took away the
last 'N' spells after choosing a random N.  That kept casting letters
sane, since letters for lost spells became invalid and those for non-lost
ones stayed the same as they were before amnesia.  But 3.4.x gave the
player the ability to swap pairs of spells, so he could make his favorites
to be the first spells, and only lose them if the random number of spells
being affected was as large as the whole list.  (Also, divine spellbook
gifts give preference to books for unknown spells; in theory, you could
use spell letter manipulation plus deliberate amnesia to make a particular
spell revert to unknown in order to improve the chance of getting a new
spellbook for it.  A bit of cleverness by a determined player but it
makes the game and/or its patron deities seem a bit dumb in the process.)

     I first implemented losing spells throughout the list, with later
spells moved forward to fill any gaps.  But that results in new casting
letters for every spell past the first lost one, potentially wreaking
havoc if a player chooses a casting letter from his own memory of the
pre-amnesia list.  So, instead of losing some spells entirely, either
from the end of the list or spread throughout, I've changed amnesia to
set the retention amount (of N spells from throughout the list) to zero,
the same as happens when it's been 20000 turns since the spell was last
learned.  Letters for all known spells stay unchanged, and forgetting
due to amnesia becomes the same as the other way of forgetting spells.
(So now a different potential clever use of amnesia occurs; a player who's
trying to a make speed ascension could get access to expired spells--to
cast in order to become confused--without waiting for 20000 turns after
reading the first book.)