Add multipliers for sub-second timing

[stevegrunwell]

I reviewed a PR at work today that included logic that took some number of seconds as an integer, then had to compute the number of milliseconds in order to interact with a Prometheus service. I realized that even if we were using this library, there would still be a $seconds * 1000 expression somewhere, which means people trying to do [albeit simple] math.

This PR adds four "multipliers", representing the most common submultiples of second in the International System of Units:

Unit	Symbol	Constant	Size
Millisecond	ms	MILLISECONDS_PER_SECOND	One thousandth (1/1000) of a second
Microsecond	µs	MICROSECONDS_PER_SECOND	One millionth (1/1000000) of a second
Nanosecond	ns	NANOSECONDS_PER_SECOND	One billionth (1/1000000000) of a second
Picosecond	ps	PICOSECONDS_PER_SECOND	One trillionth (1/1000000000000) of a second

It's unlikely that nanoseconds nor picoseconds will really get much use, but microseconds are useful when dealing with high-resolution time, and milliseconds are popular for measuring performance, microcaching, and more.