game: cache some calculations for enemies

scenes/utils/keyed_cache.gd

@@ -0,0 +1,17 @@
+class_name KeyedCache
+extends Resource
+
+var cache = {}
+
+
+func get_or_compute(key: String, compute_func: Callable):
+	if key in cache:
+		return cache["key"]
+
+	var value = compute_func.call()
+	cache["key"] = value
+	return value
+
+
+func invalidate_key(key: String):
+	cache.erase(key)