Use integers in stat tuple, optionally floats in named fields.

Thanks to Skip Montanaro and Kalle Svensson for the patches.

This is a big one, touching lots of files.  Some of the platforms
aren't tested yet.  Briefly, this changes the return value of the
os/posix functions stat(), fstat(), statvfs(), fstatvfs(), and the
time functions localtime(), gmtime(), and strptime() from tuples into
pseudo-sequences.  When accessed as a sequence, they behave exactly as
before.  But they also have attributes like st_mtime or tm_year.  The
stat return value, moreover, has a few platform-specific attributes
that are not available through the sequence interface (because
everybody expects the sequence to have a fixed length, these couldn't
be added there).  If your platform's struct stat doesn't define
st_blksize, st_blocks or st_rdev, they won't be accessible from Python
either.

(Still missing is a documentation update.)