Encapsulate the dependencies more cleanly.

also promote to its own directory, make local to nll

You should always be able to know that any `T` where `T: Idx`
can be used in a `BTreeMap` and a `FxHashMap`.

We used to store one value per RegionVid; we will soon be storing one
value per SCC.

The strategy is this:

- we compute SCCs once all outlives constraints are known
- we allocate a set of values **per region** for storing liveness
- we allocate a set of values **per SCC** for storing the final values
- when we add a liveness constraint to the region R, we also add it
  to the final value of the SCC to which R belongs
- then we can apply the constraints by just walking the DAG for the
  SCCs and union'ing the children (which have their liveness
  constraints within)

There are a few intermediate refactorings that I really ought to have
broken out into their own commits:

- reverse the constraint graph so that `R1: R2` means `R1 -> R2` and
  not `R2 -> R1`. This fits better with the SCC computation and new
  style of inference (`->` now means "take value from" and not "push
  value into")
  - this does affect some of the UI tests, since they traverse the
    graph, but mostly the artificial ones and they don't necessarily
    seem worse
- put some things (constraint set, etc) into `Rc`. This lets us root
  them to permit mutation and iteration. It also guarantees they don't
  change, which is critical to the correctness of the algorithm.
- Generalize various helpers that previously operated only on points
  to work on any sort of region element.