exhaustiveness checking sometimes fails to detect missing variants

[nikomatsakis]

I can't get a narrow test case, but the function bound_region_to_str() was missing a variant and no error was reported. The relevant case is now handled, but I added a FIXME tagged with this issue # so if you want to debug it, you can remove that variant.

[nikomatsakis]

never mind, turned out to be user error, but of a scary kind