Add List.extend and List.to_list, plus some more tests

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/obj/ty.rs

@@ -190,6 +190,7 @@ pub fn init_types() {
         List {
             // Conversion methods
             to_repr => BuiltinFunction::create("to_repr", List::to_repr, 1),
+            to_list => BuiltinFunction::create("to_list", List::to_list, 1),
 
             // Constructor
             __call__ => BuiltinFunction::create("__call__", List::do_call, 2),
@@ -203,6 +204,7 @@ pub fn init_types() {
 
             push => BuiltinFunction::create("push", List::push, 2),
             pop => BuiltinFunction::create("pop", List::pop, 1),
+            extend => BuiltinFunction::create("extend", List::extend, 2),
         },
         Str {
             // Conversion methods