make HR algorithms account for region subtyping

Currently, we consider region subtyping a failure
if a skolemized lifetime is relatable to any
other lifetime in any way at all. But a more precise
formulation is to say that a skolemized lifetime:

- must not have any *incoming* edges in the region graph
- only has *outgoing* edges to nodes that are `'static`

To enforce the latter requirement, we add edges from `'static -> 'x` for
each lifetime '`x' reachable from a skolemized region.

We now have to add a new `pop_skolemized` routine to do cleanup.
Whereas before if there were *any* edges relating to a skolemized
region, we would return `Err` and hence rollback the transaction, we now
tolerate some edges and return `Ok`. Therefore, the `pop_skolemized`
routine runs and cleans up those edges.

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