Prevent #rub from wielding an item that is already being worn
(which should narrow things down to the various types of eyewear;
other tools and weapons that go through wield_tool() can't be worn).
Fix up the wield_tool message spacing in the process.
This moves wield_tool() from apply.c to wield.c. Some plural
handling for messages is included; it is feasible to try to #rub a
"pair of lenses" or a stack of N candles.
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