various helper functions for the vba interface

New vba.py features include:
* get_memory_at
* get_memory_range
* set_memory_at

Also, the "crystal" class has a number of specialty helpers:
* crystal.walk_through_walls
* crystal.get_player_name
* crystal.get_map_name
* crystal.get_xy
* crystal.nstep (which sets memory each step by calling certain
* functions, like walk_through_walls)
* crystal.is_in_battle
* crystal.get_gender