Moved formatting and parsing of sys_time and local_time from tz.h to
date.h in order to make this functionality available to a wider
audience. Existing code does not need to change. But future code
can #include "date.h" instead of "tz.h" and need not compile tz.cpp nor
link to curl.
Formatting zoned_time remains in tz.h.
Howard Hinnant