fix: Adjust formatting for local datetime objects

[mjip]

Partially fixes #613
Fixes tests TestTOMLTest_Valid_Datetime_Local and TestTOMLTest_Valid_Datetime_Milliseconds.

Issue: #613

Explanation of what this pull request does.
Adjusts the string formatting of LocalTime structs to comply with test output (uses a T as a date/time separator, trims trailing zeros in nanosecond string).

[moorereason]

Do we want to preserve nanosecond precision while decoding?

I have a patch locally that fixed this by preserving the precision of the nanosecond value. For example, 10:32:00.000 and 10:32:00.500 would both be preserved with 3-digit precision.

The only argument I have for it is for roundtripping.

It doesn't look like BurntSushi preserves precision (10:32:00.500 → 10:32:00.5). The toml-test suite doesn't test for this case.

[pelletier]

Sorry for the late reply. I think that if there is no usability difference between the two method we should benchmark and take the fastest. @moorereason if you push that patch you have I'm happy to bench it!

[moorereason]

@pelletier, I've pushed up #626 for consideration.

[pelletier]

Thank you!