Rewrite generate_pawn_moves() and simplify evasions

Big cleanup and semplification of pawns evasions that
now are pseudo-legal as the remaining moves. This
allow us to remove a lot of tricky code.

Verified against perft: no functional change.

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

src/movepick.cpp

@@ -144,7 +144,7 @@ void MovePicker::go_next_phase() {
 
   case PH_EVASIONS:
       assert(pos.is_check());
-      lastMove = generate_evasions(pos, moves, pinned);
+      lastMove = generate_evasions(pos, moves);
       score_evasions();
       return;