<chrono>: Handle overflow cases in [time.format]

[mnatsuhara]

Observed by @statementreply.

There are two specific cases of overflow that we are not currently handling correctly:

> format("{:%Q%q}", nanoseconds::min())
< --9223372036854775808ns
> format("{:%T}", duration<float, ratio<1, 10'000'000>>{179999981568.0f})
< 05:00:00.-016384

We should also audit for other cases of overflow.

Discovered while implementing #12.