Introduce "phutil_partition()" and natural case sorting for "msortv(...)"
Ref T13546. Pull some small utility changes out of the deeper stack of "land/markers" changes.
"phutil_partition()" makes it easier to write code that loops over a list grouping elements, then acts on each group. This kind of code is not terribly common, but often feels awkward when implemented with raw primitives.
"msortv()" can support "natural" sorting, which sorts "feature1", "feature2", ..., "feature10" in a more human-readable order.
Test Plan: Ran unit tests, used new behaviors elsewhere in "arc markers" workflows.
Maniphest Tasks: T13546
Differential Revision: https://secure.phabricator.com/D21371