Add update_special_bitboards! to Ruby-facing C.

This needs to be called after making a board from a FEN string. Oops.
Add pawn attack bitboard tables.

These are going to be useful for move generation and determining if a
square is attacked.
Add a bitboard-to-index method in utils.

This is based off of Java's implementation of calculation of number of
trailing bits; better would be an implementation that assumes we only
have a single bit set.
Add an is_attacked function for boards.

All attacks are more or less cached; no move-generation calculation
required. Hooray!
Make is_attacked available to Ruby.

I don't see why it'd be useful for Ruby to be able to choose what side
is attacking, so Ruby can only ask if the side to move can attack the
index passed.
Call update_special_bitboards! after reading from FEN.

Otherwise, a bunch of methods like is_attacked become broken.
Add minimal testing for attacked?

This is really not enough to make sure the method works.