crypto/elliptic: move P-521 group logic to internal/nistec

crypto/elliptic: move P-521 group logic to internal/nistec

This abstracts the clunky and not constant time math/big elliptic.Curve
compatibility layer away from the pure fiat-backed group logic.

Change-Id: I3b7a7495034d0c569b21c442ae36958763b8b2d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/320074
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>