Follows hyperfile 0.7.0, where a checkpoint may be written as several objects and
the parted format is the default. The unit this crate reports and deletes is a
stored object rather than a checkpoint, and the delete set comes from what is on
storage rather than from arithmetic over the range.

collect and prune take Cno, the checkpoint number a caller speaks; the lists they
return name objects. Object granularity is what makes reclaim possible under the
new format at all: a checkpoint abandoned by a rollback leaves its partials
behind and the retry reuses its number, so the dead partials and the live
checkpoint share a cno.

Listing the container's prefix replaces building a key per number in the range.
Those names are wrong for a streamed checkpoint, and deleting a name that does
not exist succeeds — so the failure would have been a silent no-op. The summary
of the walked checkpoint, and the summary part of any checkpoint that still has a
live part, are kept regardless.

The minor version skips 0.6 to line up with the hyperfile release this tracks.
Requires hyperfile 0.7.