Microptimize pawns info access

Avoid indirect calling of piece_of_color_and_type(c, PAWN) and its
alias pawns(c) in the pawn evaluation loop, but use the pawns
bitboards accessed only once before entering the loop.

Also explicitly mark functions as static to better self-document.

No functional change.

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