Autopickup for pickup_types is disabled for unpaid shop goods, but
always-pick-up autopickup_exception still picked things up, as did
pickup_thrown for thrown objects. Now they'll only work in shops
for no_charge items (dropped by hero and shk didn't want, or hero
declined to sell).
costly_spot() was being recalculated for every item which passed
pickup_types. This changes autopickup to check it once for a given
autopickup operation and cache the result. [Whenever the hero moves,
a room entry check is performed. That ought to be extended to cache
costly_spot() for <u.ux,u.uy> in iflags or u.]
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