ICU: Store time in second resolution

[Flamefire]

ICU was the only backend storing subseconds in its calendar and hence reported fractional times in date_time::time() However outside of date_time::time(floor(time)) there was no way setting those milli/nanoseconds. This leads to unexpected behavior for comparisons of seemingly identical time points, especially as it depends on the backend.

Closes https://github.com/boostorg/locale/issues/221

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (741bf1d) 95.72% compared to head (1d03d20) 95.70%.

Additional details and impacted files

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