Some cleanup of std.datetime.

This is mostly some cleanup which takes advantage of some stuff like
format being pure now, but it also removes the long month name and
case-insensitive months for simple time. I don't know why it was there
(probably idiocy on my part), and it violates the format for simple time
as defined by the documentation. The functions for that are private
and don't affect anything aside from `fromSimpleTime`.