* Many of the ideas and some of the code herein is credited to
Adrian Colomitchi
* Decouple fractional decimal seconds formatting from time_of_day
formatting so that it can be more easily used elsewhere in the
future.
* Include super-second durations such as nanocenturies and
microfortnights in the class of durations that will get formatted
with fractional decimal seconds.
* If a duration is exactly representable with fractional decimal
seconds not exceeding 18 decimal fractional digits, format it
exactly. Otherwise format it to 6 fractional decimal digits
(microseconds precision). The number 18 is chosen as this is the
limit of std::ratio using 64 bits (i.e. atto).
* The above bullet implies that durations with ratio<1, 4> will now
be formatted with 2 fractional decimal digits instead of 1.
ratio<1, 8> will be formatted with 3, and ratio<1, 3> with 6.
* Unify the implementation into one C++11 implementation that works
equally well with C++14.
* Drive-by fix a couple formatting bugs dealing with negative
durations.
* Deprecate the make_time functions taking unsigned md by removing
their documentation. Also deprecate the corresponding time_of_day
constructors taking unsigned md.
* This change paves the way for future formatting improvements.
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