mpn_invert implementing ApproximateReciprocal.

[Paul Zimmermann]

       Marco,

> I finally had some time to implement and test a version of mpn_invert
> using Newton iteration, the code is attached.

congratulations, your timings are very good!

Just a minor remark: you might want to explain clearly what the function
really computes (in particular in which sense it gives an approximation
of the inverse). This will ve very useful for people who want to use this
function.

Paul