Prep for dropflag hints: refactor trans:_match to pass around MatchInput rather than ValueRef.

(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.

Author: pnkfelix