crypto/elliptic: call IsOnCurve via the interface.
https://go-review.googlesource.com/#/c/2421/ contains an unfortunate
slip where IsOnCurve is called on the CurveParams rather than the curve.
This doesn't really matter, but it's a pain for people doing tricks with
crypto/elliptic and means that 1.5 would be a regression for them
without this change.
See https://groups.google.com/forum/#!topic/golang-dev/i8OPUTYctOk