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.
In private code, I do not care. As long as we will continue to support both syntaxes, neither to me is the "right" way.

For public aliases, I think we should use the alias x = y. I find it more intuitive, and I think most people do, sorry Jonathan you are the exceptional person :)