Add special bitboard update method.
Make moves be instantiable again.
Make boards be mutable from Ruby-side, fix bug in new_from_fen.
Add make_move and update_special_bitboards to board.
Add Ruby-facing methods for make_move.
Add test for make_move.
Remove move's now defunct test for instantiation.
Add move.h's constants as Ruby constants too.
Bugfix: convert ep loc into numerical location before passing to C.
Add a bunch test methods for make_move.