* Add mnt6_753 curve
Generalize mnt6 curve model
* Add mnt4 curves
* Use resampled generators
* Calculate correct G2 cofactors
* Add fields to r1cs-std
* Add pairings
* Improve reusing of Fq/Fr among MNT curves
* Add instantiations of curves
Fix Fp6_2over3
Rebase code to current master
* Add test for recursive NIZK proof verification
* Address comments in PR
* Improve test case and port to GM17
Also fix a minor bug in to_field_vec
- contributes to #50,
- depends on #53 and builds on it,
- due to coherence & requirements of `num_traits::{Zero, One}` to implement `std::ops::Add<Self, ..>` and (resp.) `std::ops::Mul<Self, ..>`, I've had to replace the afferent `impl<'a, P: ..> (Add|Mul)<&'a Self> for Group(Affine|Projective)<P>` by direct implementations on `Self`,
- I did not have to fight the borrow checker for this conversion => I think this hints arithmetic operations are called in contexts where the operand is owned,
- hence should this end up on a merge track, we may want to open an issue to convert the `impl<'a, P:..> (Neg|Sub|..)<&'a Self> for ..<P>` trait usage to direct `impl<P:..> (Neg|Sub|..)<Self> for ..<P>`
- the `impl AddAssign for GroupAffine<P>` in curves/models/short_weierstrass_jacobian.rs is provided to fit trait bounds, and without any guarantee of suitability for any particular purpose
- and that, even though I don't think it's used.
