Switch to descending canonical sort order

[sdboyer]

#82 introduces explicit version statements within schemas. This doesn't change Thema's basic invariants - exactly one correct version number is still entailed by the combination of backwards compatibility and immutability invariants.

However, it does introduce the possibility that schemas might be declared in different orders, because version is no longer so strictly derived from a list index. Given that, i suspect the most ergonomic way of managing lineages over time will be to present the newest first, rather than last, as would generally be implied by an append-only datastructure. This descending ordering would hold for both schemas and lenses.

The effect of this would be that formatting tools should choose to output a lineage with versions [0, 0], [0, 1], and [1, 0] in the following order:

• [1, 0]
• [0, 1]
• [0, 0]

i'm not fully committed to this yet.