RFC: Single-entry / multiple-exit regions for borrows

[Rendered](https://github.com/zwarich/rfcs/blob/seme-regions/active/0000-seme-regions.md)
I first came up with the more technical definition below and then tried to make a plain English counterpart.

What I was attempting to capture was the part about reentering through Entry. For example, in a program fragment like

``rust
loop {
    [Entry]
    [Exit]
}
```

there is allowed to be a path from Entry to Exit, back around the outer loop that reenters Entry, without all of the vertices on that path being in the region. On second thought, it seems that this would be fine:
> If V is in R and P is a path from Entry to V that does not contain Entry multiple times, then every vertex of P is in R.