This commit removes code duplication where we had 2 versions of a
`flatten` utility. Moreover this change results in queries using
a non-recursive version of `flatten` which should result in a better
performance of query refresh operations.
PR Close#29547
Google3 detected circular references here, so splitting up this rather hodge-podge list of functions into slightly better organizational units.
PR Close#28382
Pawel Kozlowski
Ben Lesh
Misko Hevery
Victor Berchet