Extractors for value class trees.

Salvaged from work six months ago on value class bugs. Seemed like
a shame to lose it (look at the "after" in posterasure.) There may
be some modest overlap with some other tree extractors; I suggest
that tree extractors are not amenable to one-size-fits-all-ness,
and modest overlap should be tolerated, maybe even embraced.

I admit ignorance as to what happend with t6028.check.