Refactor liveness to work on the CFG and use dataflow

[nikomatsakis]

Now that @pnkfelix has landed CFG and is about to land dataflow, it's time for liveness code to be rewritten to use it.

Currently liveness has two responsibilities:

• issuing warnings for dead writes
• checking that functions always return a value or fail

Probably liveness as a pass should go away and be moved into borrowck.

cc @zwarich

[steveklabnik]

I think frontend is the right tag?

[arielb1]

This is now a part of the MIR issue.