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

[Rendered](https://github.com/zwarich/rfcs/blob/seme-regions/active/0000-seme-regions.md)
The idea of counting uses of Entry seems a little odd. Could you not use something like the following:

> For every pair of vertices V and U in R every path from V to U that contains vertices not in R also contains Entry.