Refactor encumbrance

[ymber]

Is your feature request related to a problem? Please describe.
The encumbrance handling code is disgusting. It's a mountain of hacks piled on top of a bad implementation from before DDA and its complexity is blocking improvements.

Describe the solution you'd like
Refactor the code handling coverage and encumbrance. It's possible this is a set of linked problems and the body part rework we need for proper injury modeling should come first. Encumbrance should be cached and recalculated whenever it may change. Calculating it every time the values are needed would be terrible for performance. The new system has to allow for the UI improvements jbtw made last time this code got major changes to work.

We may want to give Character a std::set<std::list<std::pair<layer, item_location>>> clothing_data storing a list of <item_location, layer> pairs for each body part. A separate structure is a cache storing ints for encumbrance per bodypart and is updated every time clothing_data is modified or an item pointed to by an item_location in it is modified.