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.
> because that could easily lead to weird behavior and nasty bugs if the clocks are mixed.

Well, from my understanding the "coarse" time is derived from the same clock as the monotonic time, but just captured once every tick by the OS instead of on demand. This means it's possible to read the monotonic time, and then read the coarse time, and get a negative delta. I think at the very least, we should warn people that we do not guarantee different clocks are exactly in sync.

Just a recommendation to only use one clock consistently would be good.