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

[Rendered](https://github.com/zwarich/rfcs/blob/seme-regions/active/0000-seme-regions.md)
it's unclear to me what this fragment is demonstrating. q is an immutable binding, so it has to be initialised once-and-only-once subsequently, right? is the idea that MEME would allow the q= lines in each branch to borrowcheck because they're each an entry to the same region (and each of the drop(p) is an exit from the previous region?)