Check the equality of EdwardsPoints in the projective coordinates

[ebfull]

I stumbled on this sentence at ristretto.group:

Equality testing of points on the Edwards curve requires comparing to affine coordinates, which requires an expensive inversion. However, testing whether two points lie in the same coset can be done in projective coordinates, making it actually easier than equality testing in the original non-quotient group.

which led me to check the code here and notice this could be optimized. 😺

[ebfull]

Test failures are unrelated to this PR I think.

[hdevalence]

Looks like the build failure is related to rust-random/rand#702

[ebfull]

Note that the current implementation of is_small_order seems to invoke this to compare against the identity, requiring several inversions.

[hdevalence]

Thanks!