Assorted trivial cleanups

- Cleanups by Alain
- Group king attacks and king defenses
- Signature of futility_move_count()
- Use is_discovery_check_on_king()
- Simplify backward definition
- Use static asserts in move generator
- Factor a statement in move generator

No functional change

src/pawns.cpp

@@ -108,7 +108,7 @@ namespace {
         // A pawn is backward when it is behind all pawns of the same color on
         // the adjacent files and cannot safely advance.
         backward =  !(neighbours & forward_ranks_bb(Them, s + Up))
-                  && (stoppers & (leverPush | blocked));
+                  && (leverPush | blocked);
 
         // Compute additional span if pawn is not backward nor blocked
         if (!backward && !blocked)