Second take at unifying bitboard representation access

This patch is built on Tord idea to use functions instead of
templates to access position's bitboards. This has the added advantage
that we don't need fallback functions for cases where the piece
type or the color is a variable and not a constant.

Also added Joona suggestion to workaround request for two types
of pieces like bishop_and_queens() and rook_and_queens().

No functionality or performance change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>