Make g->z private; reduce mon_at() calls.
Make monster::posx/posy private.
Use a map to make mon_at() 144% faster.
Fix more g->z accesses so the z_at map is used.
Fix z_at indices when removing a zombie.
Make player::has_trait() faster.
Clean up monster::plan().
Const strings for vehicle methods.
Use a bitset for mtype::has_flag.
Merge monster tracking refactor and performance enhancements.

Conflicts:
	disease.cpp
	wish.cpp
Prevent lockup due to inability to find a place to stash stairs.