crypto/ecdsa: clean up ECDSA parsing and serialization paths

crypto/ecdsa: clean up ECDSA parsing and serialization paths

Check for invalid encodings and keys more systematically in
ParseRawPrivateKey/PrivateKey.Bytes,
ParseUncompressedPublicKey/PublicKey.Bytes, and
fips140/ecdsa.NewPrivateKey/NewPublicKey.

Also, use these functions throughout the codebase.

This should not change any observable behavior, because there were
multiple layers of checks and every path would hit at least one.

Change-Id: I6a6a46566c95de871a5a37996835a0e51495f1d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/724000
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>