Add support for Uint and Uint64

[creiht]

I would like to be able to log unsigned ints as part of the structured data.

[akshayjshah]

Makes sense. I'm happy to add this, but it'll have to wait until after the 1.0 release work is done. If you have time to work on a PR, we can land it earlier - ping this issue and I'll suggest an implementation.

[creiht]

I can take a stab at it. I had looked at the code a bit, and it didn't look too bad. If it will require anything besides following the pattern for what has been done for int and int64, let me know.

[akshayjshah]

Sounds great! Unsigned int handling should follow the pattern for int and int64, casting to/from int64 to re-use Field.ival.

You may want to hold off until the end of the day, though, since we're in the midst of landing a bunch of large refactorings.