9ef61f1ee0
(already thumbs-upped pre-rebase by nikomatsakis) The refactoring here is trivial because `trans::datum::Lvalue` currently carries no payload. However, future commits will start adding a payload to `Lvalue`, and thus will force us either 1. to thread the payload through the `_match` code (a long term goal), or 2. to ensure the payload has some reasonable default value.