Use MoveList also in Position::move_is_pl_slow()

And rename it in Position::move_is_legal()

No functional change.

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

src/movegen.h

@@ -36,6 +36,8 @@ enum MoveType {
 template<MoveType>
 MoveStack* generate(const Position& pos, MoveStack* mlist);
 
+/// The MoveList struct is a simple wrapper around generate(), sometimes comes
+/// handy to use this class instead of the low level generate() function.
 template<MoveType T>
 struct MoveList {