Add ClockType enum to core.time for issue# 13433.

This adds an enum for indicating which type of clock to use when it's
appropriate for a time function to have multiple options for the source
clock. Currently, it only supports "normal" and "coarse," but we may add
more in the future, since some systems do support others (e.g
clock_gettime has several other options depending on the system).

This is in support of https://github.com/D-Programming-Language/phobos/pull/2584.
Can you specifically number these? What I see happening is sometime in the future, e.g., FreeBSD will obtain bootTime clock, and then where does it go? It would have to go at the end, which means the ordering under FreeBSD will be incorrect.

Also, note that if two systems want to share a timestamp (an eminently serializable item), the numbers will be different!