add Date, Time, DateTime, and PosixTz

[billzez]

I've been using this code for a few months, and decided its probably time to upstream it.

• adds functionality provided by libc's mktime, localtime, gmtime, timegm, asctime, strftime, etc...
• parse posix timezone strings
• parse http "Date" fields
• parse ISO 8601 strings

[Vexu]

Weekdays should be numbered from 1 to 7 starting with monday.

[billzez]

This is a POSIX vs ISO convention debate that I would rather not get into. These algorithms are based upon what is typically done in libc. And since Zig eventually hopes to implement it's own libc, the POSIX way made more sense to me.