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

[Rendered](https://github.com/zwarich/rfcs/blob/seme-regions/active/0000-seme-regions.md)
This sounds tautological. "If [...] P is a path [...] that does not leave R [...], then every vertex of P is in R". Surely the condition "P is a path [...] that does not leave R" is identical to "P is a path where every vertex of P is in R", which makes this "If P is a path where every vertex of P is in R, then every vertex of P is in R".