now that the analysis is only using the regular liveness shape, we don't
need to store it transposed, and thus don't need the container where it
was stored: the liveness context would be alone in the polonius context.

we thus remove the latter, and rename the former.

we may need to traverse the lazy graph multiple times:
- to record loan liveness
- to dump the localized outlives constraint in the polonius MIR dump

to do that we extract the previous loan liveness code into an abstract
traversal + visitor handling the liveness-specific parts, while the MIR
dump will be able to record constraints in its own visitor.

now that we need to hold the graph for MIR dumping, and the associated
data to traverse it, there is no difference between the main context and
diagnostics context, so we merge them.