Step up the error reporting on unexpected NULLs during collapse

Step up the error reporting on unexpected NULLs during collapse

The collapser became so complex that it is practically impossible to debug
when things go sideways. Expand the rudimentary "just check root identifier"
to a comprehensive check of all identifier groups, at the expense of some
performance. This builds on top of the {nullchecks} metadata collected in
5ff6d603, generating maximally unrolled definedness checks which cause an
early return with no results and flagged unexpected-null-positions

The commit itself is relatively straightforward, though the meager changeset
in lib/ is misleading: the damned thing took almost 8 days to fully think
through and implement with several detours for bugfixing :/