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.
Wiki links to what? And while it's a fair amount of text, I think that it does a decent job of explaining, and I'm not sure how I could change it to be much shorter without losing information. But I've never been very good at being concise.