Carry a rounded fractional second into the next second

[SashaMIT]

Summary

format_time and format_datetime rounded a fractional second by printing an extra digit. time(0, 0, 1, 999999) with pattern ss.SSS returned 01.1000.

The timestamp is now rounded to the pattern's S width before the fields are formatted, so that second carries and the fraction stays the requested width (02.000). A pattern with no S field is left alone, and a quoted S stays literal text.

Test plan

• time(0, 0, 1, 999999) formatted as ss.SSS is 02.000, and ss.S is 02.0
• The same value formatted as ss stays 01, and ss 'S' stays 01 S
• 23:59:59.999999 formatted as HH:mm:ss.SSS rolls to 00:00:00.000
• 2020-01-01 23:59:59.999999 formatted as yyyy-MM-dd HH:mm:ss.SSS rolls to 2020-01-02 00:00:00.000
• time(0, 0, 1, 234600) formatted as ss.SSS is 01.235 (the fraction rounds, the second does not carry)
• Existing fractional-second cases still pass (799 microseconds as SSSS is 0008, 34567 as SSSS is 0346)