[ 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.