Templetize functions to get pieces by type

Templetize functions to get pieces by type

Use a single template to get bitboard representation of
the position given the type of piece as a constant.

This removes almost 80 lines of code and introduces an
uniform notation to be used for querying for piece type.

No functional change and no performance change.

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