<chrono>: Fix hh_mm_ss subsecond formatting for floats

[eldakesh-ms]

Before, the same formatting string was used for floats and integrals.
This meant that large floats were formatted using exponent notaion and
small floats were not, and it also meant there was an extra period in a
time, as the subseconds could be fractions of a subsecond (say .4
nanoseconds). Now if the subseconds are floats, we force fixed
formatting to get the right number of leading zeroes and a precision of
0 to round off fractions of subseconds.

[eldakesh-ms]

Per discord discussion with @statementreply: Currently this rounds, but after some deliberation truncation probably makes more sense. The spec is a bit ambiguous on this. If we do rounding, we need to pull the subsecond into the seconds (if it rounds up to a whole second), and this can mean that a second becomes a minute and a minute becomes an hour. I think truncation is the easiest solution, but maybe I a missing something.