[bitboard, core] Make library getters const; parameterize Library attributes

- Add chessfriend_core::Color::NUM
- All the library getters can be const.
- Use the constants from the core library to define the length of the slices in
  the Library struct.

core/src/colors.rs

@@ -11,7 +11,11 @@ pub enum Color {
 }
 
 impl Color {
-    pub const ALL: [Color; 2] = [Color::White, Color::Black];
+    /// Number of colors
+    pub const NUM: usize = 2;
+
+    /// Slice of all possible colors
+    pub const ALL: [Color; Color::NUM] = [Color::White, Color::Black];
 
     pub fn iter() -> impl Iterator<Item = &'static Color> {
         Color::ALL.iter()