Introduce some new standard library modules

modules/collection/src/main/resources/list.mus

@@ -15,7 +15,7 @@ function _flatten(list: list, output: list) {
 }
 
 extend list as this {
-    function toFlat() {
+    function flatten() {
         return flatten(this);
     }
 
@@ -23,6 +23,16 @@ extend list as this {
         return (this as item ^ item % item == value).size > 0;
     }
 
+    function indexOf(value) {
+        this as (item, index) ^ {
+            if(item == value) {
+                return index;
+            }
+        }
+
+        return -1;
+    }
+
     function join(separator: string = ", ") {
         output = ""
         this as (item, index) ^ {