Speedup item::best_pocket ~100% (2x as fast) and misc other wins too

[akrieger]

Summary

None

Purpose of change

Profiling of unit tests (esp starting_items) after recent changes from @BrettDong showed some weird inefficiencies. Besides the usual stdlib inefficiencies with copying instead of moving or referencing, there was what looked like a glaring 2x issue in best_pocket. Code at the top was calling can_contain( it ) and exiting 'early' if it returned false. That function essentially just loops through all the contents calling can_contain( it ) on each one. That is not cheap, though, and the subsequent loop repeats basically the same calls on the same contents but with more logic around it. This doesn't seem like a true fast early exit, just wasted cycles, and accounts for ~15-20s out of 240s of test runtime for starting_items tests. Plus it's live code.

Describe the solution

Delete the probably redundant can_contain( it ) call. Fix some list insert calls to be splices, and use a reference to a std::map instead of a copy when calling made_of().

Describe alternatives you've considered

Testing

Tests still pass.

Additional context

After/Before of starting_items test with these changes @ 100 samples/second shows ~1800 sample reduction in test runtime, basically all from Character::migrate_items_to_storage without cost shift to elsewhere.