Cherry pick a nice refactoring from the auto-filter branch.
WIP commit: use interpreter functions to convert a test variable's internal representation into a value supplied by the client code suitable for building a test case from, or using as-is. This will open up the possibility of slicing partial test vectors that include singleton values.
OK, now paasing singleton values through slicing when doing filtering. Great.
Merge in passing of singleton values in filter slices, this will help sort out this branch's implementation woes.
Now applying autofilters in subtrees as well as at the top-level node.