Add an invert_vartime method to ConstMontyForm

[TitouanReal]

Now that the inv method is constant time, it could be useful to have a vartime alternative.

This issue is a feature request for a method like this:

let x: ConstMontyForm<MOD, SAT_LIMBS> = ...
x.invert_vartime()

that computes the inverse of x in the field of cardinal MOD, constant time in regards x but vartime in regards to MOD.

[tarcieri]

#731 adds the only implementation of invert_vartime we can support using the safegcd algorithm, which varies the number of iterations by the value being inverted.

This isn't quite what you're asking for in the description: unfortunately it's constant-time with respect to MOD, but variable time with respect to x, which is unfortunately the only combination we can support. It would be nice to be able to support the opposite, especially for things like elliptic curve field inversions where the modulus is public, but that only works for odd inputs, not even ones, and so field inversions are typically implemented using the fully constant-time version, at least for the case the input values to be inverted are expected to be potentially secret.

[TitouanReal]

I wanted this in the hope of gaining some performance in elliptic curve field inversions.
Guess I'll go on the hunt for performance improvements in my implementation then. I got a 31-fold performance loss to reduce now, I better go back to work!

[tarcieri]

#634 is probably the best option for improving performance of field inversions