We've been a bit lazy about adding line numbers to all ast nodes.
This is an inconvenience when the tracebacks are bad/wrong, and it's
also causing issues for testing code that wants to look at the line
at which something failed.

So I added line numbers to a whole bunch more nodes.  There are still
quite a few that don't have line numbers, but many of them are harmless,
so I tried to identify the ones that matter.  I also added some assertions
to make sure that bad (non-positive) line numbers don't escape to the user.