Use prefixed seconds when possible for chrono-equivalent units

[chiphogg]

See this link for current behaviour:

https://godbolt.org/z/Kxe8vrh48

We could create a utility template that says, effectively, "if any of these prefixes produces a quantity-equivalent unit, then use that instead of just the unit-and-mag".

Of course, once #85 gets fixed, we won't see [UNLABELED_UNIT]; we'll see something like [1 / 1000000000] s. But the prefixes would be a nicer user experience.