It is causing problems in (some) downstream crates, and we won't want
these annotations in the codebase anyway if a downstream crate is
collecting metrics. We'll be replacing this with the cost model binary
in https://github.com/zcash/halo2/pull/225 shortly.
The `Curve` trait is now `CurveExt: group::prime::PrimeCurve`, and
`CurveAffine` is now `CurveAffine: group::prime::PrimeCurveAffine`.
There is no `CurveAffine` trait in `group`, and it's a widely-used
trait in this crate, so we don't rename it to `CurveAffineExt`.