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.
profiling maybe? I don't know. All it says on FreeBSD's man page is

    CLOCK_PROF for time that increments when the CPU is running in user or kernel mode

I don't exactly have a lot to go on for what the stray FreeBSD clocks do: http://www.freebsd.org/cgi/man.cgi?query=clock_gettime