Add immutable collections hierarchy with runtime/compiler/docs/tests

docs/List.md

@@ -1,6 +1,7 @@
 # List built-in class
 
 Mutable list of any objects.
+For immutable list values, see [ImmutableList].
 
 It's class in Lyng is `List`:
 
@@ -196,4 +197,5 @@ It inherits from [Iterable] too and thus all iterable methods are applicable to
 
 [Range]: Range.md
 
-[Iterable]: Iterable.md
+[Iterable]: Iterable.md
+[ImmutableList]: ImmutableList.md