More robust stacking

A better (in my opinion) version of stacking that is more robust and (in
my opinion) more intuitive. The items store their stackability and stack
size, rather than a std::map. This version also has the advantage of
actually saving the stacksize between sessions as well as allowing for
more than one item to be picked up or dropped at one time.

Keep in mind that for this version to work well, since it relies on
strncmp, similar items with different levels of effectiveness (better
medkits) will need different names (First Aid Ointment->Medkit->Large
Medkit->Healing Nanite Paste->etc).