Add a tuple timestamp

[frankmcsherry]

This PR adds a Timestamp implementation for (T1, T2), following Rust's lead that this is a lexicographically ordered sequence, rather than a partially ordered product (use order::Product for that).

I think the path summary representation is .. pessimistic, but not incorrect. It allows summaries to both coordinates, which are both applied. In principle, advancing the outer coordinator could zero the inner coordinate, so (summ1, summ2) could be "if summ1 is non-zero apply summ1 and set the second coordinate to T2::minimum() subjected to summ2".

I didn't do that, and just applied the summaries to each coordinate, promising more of an advance than is strictly required.

In writing this up, I took the opportunity to relayout order.rs which, not having a second implementation, lacked for organization.