Add EntityIndex::EMPTY

src/ecs/entity_index.rs

@@ -58,8 +58,10 @@ pub struct EntityIndex<T: Copy + Default> {
 }
 
 impl<T: Copy + Default> EntityIndex<T> {
+    pub const EMPTY: EntityIndex<T> = Self::new();
+
     #[must_use]
-    pub fn new() -> Self {
+    pub const fn new() -> Self {
         Self {
             dense: Vec::new(),
             alive_count: 0,