Being hit by Fire Brand now stops the sliming process if you're
being turned into green slime. And throwing an artifact up will give
artifact hit effects when it falls back down, making it possible to
hit yourself with Fire Brand (among other things). Hitting yourself
with Vorpal Blade or the Tsurugi of Muramasa will never behead or
bisect you; hitting yourself with Stormbringer will drain levels.
(I hope the latter doesn't make it too easy for players to manipulate
their experience level.)
This doesn't add artifact_hit() handling to mthrowu. If monsters
ever start throwing artifacts that will need to be added.
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