Add a helpers.FSTime class to handle filesystem timestamps and use it.

Add a helpers.FSTime class to handle filesystem timestamps and use it.

Add helpers.FSTime class to handle filesystem timestamps, and avoid
scattered conditionals.  At the moment, when _helpers.lstat is
available, FSTime stores timestamps as integer nanoseconds, otherwise
it uses the Python native stat representation (i.e. floating-point
seconds).  Operations like from_stat_time, from_timespec, to_timespec,
and approx_secs handle the appropriate conversions.

Add FSTime tests, and test both representations when the platform has
_helpers.lstat.