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/types.h

@@ -341,7 +341,7 @@ inline Score operator*(Score s, int i) {
   return result;
 }
 
-/// Multiplication of a Score by an boolean
+/// Multiplication of a Score by a boolean
 inline Score operator*(Score s, bool b) {
   return Score(int(s) * int(b));
 }