Enforce that Fr of Engine is the scalar for curve points

[ebfull]

In bellman, I want to write:

pub struct Point<C: CurveProjective>(pub C);

impl<C: CurveProjective> Copy for Point<C> { }

impl<C: CurveProjective> Clone for Point<C> {
    fn clone(&self) -> Point<C> {
        *self
    }
}

impl<C: CurveProjective> Group<C::Engine> for Point<C> {
    fn group_zero() -> Self {
        Point(C::zero())
    }
    fn group_mul_assign(&mut self, by: &C::Scalar) {
        self.0.mul_assign(by.into_repr());
    }
    fn group_add_assign(&mut self, other: &Self) {
        self.0.add_assign(&other.0);
    }
    fn group_sub_assign(&mut self, other: &Self) {
        self.0.sub_assign(&other.0);
    }
}

However, this doesn't typecheck because the compiler cannot know that the Engine's Fr type (as dictated by the Group trait) is the same as the Scalar type of the CurveProjective point. This can be solved with a where bound over the trait (for now), but it is generally a good idea for this to be constrained in pairing.

[ebfull]

Actually, it can't be solved with a where bound yet in Rust. rust-lang/rust#20041

So, I think we need this in pairing.

[burdges]

Amusingly, rustdoc itself already describes constraints like yours by an equality in the where clause.

[daira]

ACK

[ebfull]

@bmerge r+

[bmerge]

📌 Commit 471db6a has been approved by ebfull

[bmerge]

⌛ Testing commit 471db6a with merge 36ad8b6...

[bmerge]

☀️ Test successful - pairing-linux32-auto, pairing-linux64-auto, pairing-windows32msvc-auto, pairing-windows64msvc-auto
Approved by: ebfull
Pushing 36ad8b6 to master...