Convert dataflow (and liveness, and region-inference) to use a common graph abstraction

[nikomatsakis]

A needed refactoring for borrowck/liveness/etc to share code. Also to make dataflow more comphrensible and to address performance limitaitons with the current impl.

[nikomatsakis]

Clearing 0.7 milestone.

[thestinger]

Triage bump, the transition to using this still needs to be completed.

[emberian]

To my knowledge, @pnkfelix is actively working on this.