mod to symmetric range

[Paul Zimmermann]

       Hi Torbjörn,

>   Efficient code at the mpz level is attached.  I'd appreciate if you
>   Paul could review my code skeptically and if you Roman could test
>   it!
> 
> I fixed some problems.  Hopefully final code attached.

I did skeptically review the code (but not test it). See the attached file.
My comments are prefixed by PZ.

Thank you!
Paul
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