[core] Use the matches! macro to calculate the value of Shape::is_promotable

I learned about this macro a little while ago and it's better than writing out
a match block by hand, and also doesn't require static or const data, like the
previous implementation did.

core/src/shapes.rs

@@ -71,7 +71,7 @@ impl Shape {
 
     #[must_use]
     pub fn is_promotable(&self) -> bool {
-        Self::PROMOTABLE_SHAPES.contains(self)
+        matches!(self, Self::Knight | Self::Bishop | Self::Rook | Self::Queen)
     }
 
     #[must_use]