Convert SysTime's examples to ddoc-ed unittests.

This converts `SysTime`'s examples to ddoc-ed unit tests, whereas previously, most of them had the example duplicated in a unittest block, since putting a ddoc comment on a unittest block didn't used to put it in the ddoc of the previous function as an example.

Hopefully, reviewing this isn't too bad, but the diff isn't quite as straightforward as I'd hoped. Most of the cases involve simply removing the example in the ddoc and adding `///` to the unittest block, but a few required moving the examples to a separate unittest block or even reorganizing the examples a little bit (in a couple of cases, the examples were even duplicated in the tests). The end result should be cleaner though, and it definitely reduces the number of lines that the examples take up.

I'll take care of the other examples in std.datetime in future pulls. This is just for those in `SysTime`.