Millisecond accuracy

[djcsdy]

Fractional seconds are all very well but a number can't represent 1 millisecond accurately because 0.001 isn't easily expressible as a sum of powers of two.

Users of this library probably expect millisecond accuracy but not submillisecond accuracy so:

• Add a milliseconds field to Timestamp, so it's not necessary to represent milliseconds as fractions of a second.
• Replace reference seconds with reference milliseconds.
• Consider using bigint for reference milliseconds to ensure a wide range of valid dates.