add generics

test/fixtures/valid/containers.ls

@@ -6,8 +6,8 @@ more_codes := [...codes, 'failed' => 500]
 
 empty_numbers: []i32: []
 empty_codes: [str]i32: []
-optional_numbers: []i32?: nil
-optional_elements: []{i32?}: [nil, some 1]
+optional_numbers: []i32?: |.none
+optional_elements: []{i32?}: [|.none, 1->|.some]
 
 Options :: &{
     name: str: 'default'