rust-discovery: exposing sampling to python Differential Revision: https://phab.mercurial-scm.org/D6425

rust-discovery: takefullsample() core implementation take_full_sample() browses the undecided set in both directions: from its roots as well as from its heads. Following what's done on the Python side, we alter update_sample() signature to take a closure returning an iterator: either ParentsIterator or an iterator over the children found in `children_cache`. These constructs should probably be split off in a separate module. This is a first concrete example where a more abstract graph notion (probably a trait) would be useful, as this is nothing but an operation on the reversed DAG. A similar motivation in the context of the discovery process would be to replace the call to dagops::range in `add_missing_revisions()` with a simple iteration over descendents, again an operation on the reversed graph. Differential Revision: https://phab.mercurial-scm.org/D6424