added performance test
reduce grid size to speed up tests
raise InvalidGrid in constructor
raises an error if Grid#at(row, col) is an invalid position
set_pos and get_pos
refactored grid navigation
Grid#move_north
refactored everything using enumerators
combine enumerators
working combined enumerators!
more tests
refactored enumerators
all grid's words
removed unecessary code in enum_builder
final version