First part of changes for better unexpected NULL reporting

First part of changes for better unexpected NULL reporting

Doing things in two parts to make it easier to reason about. This part only
changes the collapse_map visitor to collect more metadata, and uses a bit
of it to elide a couple of // ops.

Additionally we are now feeding the stash-arrayref to the collapser at all
times, it will become clear way in the next commit (where all the real
changes are)

Read under -w