Instead, thread around `Option<CodeExtent>` where applicable.

Make a `CodeExtent<'tcx>` be something allocated in an arena
instead of an index into the `RegionMaps`.

Instead of requesting the region maps for the entire crate, request for
a given item etc. Several bits of code were modified to take
`&RegionMaps` as input (e.g., the `resolve_regions_and_report_errors()`
function). I am not totally happy with this setup -- I *think* I'd
rather have the region maps be part of typeck tables -- but at least the
`RegionMaps` works in a "parallel" way to `FreeRegionMap`, so it's not
too bad. Given that I expect a lot of this code to go away with NLL, I
didn't want to invest *too* much energy tweaking it.

This was a pretty narrow test to start with, and it's kind of a pain to
update it. Not worth the trouble IMO.