Update to logger due to fracSec deprecation in std.datetime.

I also fixed it so that it casts to DateTime rather than calling all of
the various properties on SysTime individually, since that's more or
less equivalent to casting to DateTime and getting the property off of
it for each call. I probably never should have put those properties on
SysTime in the first place, though it is more user-friendly in the cases
where you don't care about efficiency. I expect that we _do_ care about
efficiency in the logger however.

std/experimental/logger/core.d

@@ -168,16 +168,17 @@ private string parentOf(string mod)
 /* This function formates a $(D SysTime) into an $(D OutputRange).
 
 The $(D SysTime) is formatted similar to
-$(LREF std.datatime.DateTime.toISOExtString) expect the fractional second part.
-The sub second part is the upper three digest of the microsecond.
+$(LREF std.datatime.DateTime.toISOExtString) except the fractional second part.
+The fractional second part is in milliseconds and is always 3 digits.
 */
 void systimeToISOString(OutputRange)(OutputRange o, const ref SysTime time)
     if (isOutputRange!(OutputRange,string))
 {
-    auto fsec = time.fracSec.usecs / 1000;
+    const auto dt = cast(DateTime)time;
+    const auto fsec = time.fracSecs.total!"msecs";
 
     formattedWrite(o, "%04d-%02d-%02dT%02d:%02d:%02d.%03d",
-        time.year, time.month, time.day, time.hour, time.minute, time.second,
+        dt.year, dt.month, dt.day, dt.hour, dt.minute, dt.second,
         fsec);
 }