crypto/elliptic: split up P-256 field and group ops

crypto/elliptic: split up P-256 field and group ops

This makes Gerrit recognize the rename of the field implementation and
facilitates the review. No code changes.

For #52182

Change-Id: I827004e175db1ae2fcdf17d0f586ff21503d27e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/390754
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>