crypto/ecdsa: implement deterministic and hedged signatures

crypto/ecdsa: implement deterministic and hedged signatures

For the future, some test vectors we should generate and then share
through Wycheproof or CCTV:
 - A private key with a leading zero byte.
 - A hash longer than the modulus.
 - A hash longer than the P-521 modulus by a few bits.
 - Reductions happening in hashToNat and bits2octets.

Fixes #64802

Change-Id: Ia0f89781b2c78eedd5103cf0e9720630711c37ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/628681
TryBot-Bypass: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>