Checkpoint rust-cfg in *WORKING* STATE.

This includes a nasty ExprParen policy hack that nmatsakis does not like.
Hopefully it can be removed entirely.
Placate `make tidy` by adding line breaks where appropriate.
checkpoint some tests that should eventually be folded into our unit tests (i think).
Checkpoint: simplify story for change to expr_use_visitor.

In particular, simplify story for change to expr_use_visitor's
handling of ExprParen by getting rid of the need for per-client
specialization of `consume_paren_handling` method (and thus eventually
removing that method entirely, once I work out whether I need to stick
with the policy encoded here, or if I can adopt an even simpler policy
that removes the special-case handling of `if_there_are_adjustments`).
Simpler Consume ExprParen policy that will simplify the code overall.
Removed now unnecessary consume_paren_handling policy.

Drive-by cleanup: Simplify `fn consume_expr` of `ExprUseVisitor`.
Some easy code cleanup, pre-prep for posting a PR.