gather some statistics

[nikomatsakis]

There are a few things I'd love to know about the clap-rs input:

For example, what are the max in- and out-degrees that we observe in the subset graph at any point?

Similarly, when a region does dead, what are the in- and out-degrees it has at that time?

[nikomatsakis]

Probably the easiest way to do this is to modify the inspect_batch closure, which is executed for each of the final subset relations:

https://github.com/rust-lang-nursery/borrow-check/blob/a8282fa7331fd6224b99486ebb7aaa97e83ca2a0/src/output/timely.rs#L205-L220

we could then update the in/out counts for each of the regions involved at each point. We'd have to add some more fields to Output