Adding millisecond precision to SnowflakeTimestamp utility

[AndyTempel]

The current utility only allows second precision and maybe to some people millisecond precision will come in handy since Discord timestamps are in milliseconds.

[CarsonHoffman]

Instead of introducing the modulus operation and passing both parameters to time.Unix, it might just be easier to pass in timestamp*1000000 to the nanoseconds field and 0 to the seconds field. This is the pattern you'll often see with this function—keeping the parameters mutually exclusive—and takes a bit of load off reading the function (as well as performance with the modulus, as subtle as it may be).

[AndyTempel]

Yup, you're correct. I was going by (outdated) benchmarks and didn't test them my self if they still stand.

[ewohltman]

Could this PR be changed to merge into the develop branch of this repo instead of master as per the Contributing guidelines?

[AndyTempel]

Branch changed.

[CarsonHoffman]

I went ahead and added a quick unit test since we were changing how the function worked (which probably should've been there from the start). Thanks for the PR!