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.
This _isn't_ guaranteed to be faster. It's just guaranteed to give you a `SysTime` whose fractional seconds are zero, so that behavior will be guaranteed, and on non-FreeBSD systems we _probably_ can use a function for that which is faster for that even though the system doesn't have a clock for it like FreeBSD does (e.g. I believe that `gettimeofday` is faster than `clock_gettime`, but I'll have to test that), but I certainly can't guarantee that giving only second-level precision will be faster.

`second` only exists because of FreeBSD's special clock, and it seemed like it would be easy to fake on other systems and that we could _probably_ make it faster than getting the normal precision (but not guarantee it), whereas with `coarse`, the idea is to get the time faster if we can but still provide decent precision. In neither case, however, can I actually guarantee that I'll be able to get the time faster on a particular system that doesn't have a special clock set up for it.