Replace covariant PartialOrd with reborrow()

[frankmcsherry]

Batch containers have required that their items are comparable across lifetimes. This seems to confound Rust when presented with the Ord constraint in addition. Another option is to remove the constraint, and require that the contain know how to reborrow its items, narrowing their lifetimes.

At the moment this is a thing to consider; there are other ways to achieve the same goal we think (e.g. removing Ord instead).

cc: @antiguru

[antiguru]

Looks good, thanks!