After some thinking, I decided to move all of this across onto

After some thinking, I decided to move all of this across onto
the branch.  I don't think it can break code, so the only risk
is introducing new bugs.  In that vein, can I ask -checkins readers
who have time to carefully look this over, check my refcounts &c?
TIA.

backport my checkin of
    revision 1.5 of structseq.c

Apply (my) patch:

[ 526072 ] pickling os.stat results round II

structseq's constructors can now take "invisible" fields in a dict.
Gave the constructors better error messages.
their __reduce__ method puts these fields in a dict.

(this is all in aid of getting os.stat_result's to pickle portably)

Also fixes

[ 526039 ] devious code can crash structseqs

Thought needed about how much of this counts as a bugfix.  Certainly
#526039 needs to be fixed.