Optimize generate_piece_blocking_evasions()

Rewrite as in generate_piece_moves() using a for
loop instead of the slower serializing of the
bitboard with pop_1st_bit()

This will allow also to merge with generate_piece_moves()
when we will drop legality constrain on generate_evasions()

Generated moves are not changed, but are generated in a
different order, this changes the number of nodes at fixed
depth test.

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