crypto/x509: rewrite certificate parser

Replaces the encoding/asn1 certificate parser with a
x/crypto/cryptobyte based parser. This provides a significant increase
in performance, mostly due to a reduction of lots of small allocs,
as well as almost entirely removing reflection.

Since this is a rather large rewrite only the certificate parser is
replaced, leaving the parsers for CSRs, CRLs, etc for follow-up work.
Since some of the functions that the other parsers use are replaced
with cryptobyte versions, they still get a not insignificant performance
boost.

name                           old time/op    new time/op    delta
ParseCertificate/ecdsa_leaf-8    44.6µs ± 9%    12.7µs ± 4%  -71.58%  (p=0.000 n=20+18)
ParseCertificate/rsa_leaf-8      46.4µs ± 4%    13.2µs ± 2%  -71.49%  (p=0.000 n=18+19)

name                           old allocs/op  new allocs/op  delta
ParseCertificate/ecdsa_leaf-8       501 ± 0%       164 ± 0%  -67.27%  (p=0.000 n=20+20)
ParseCertificate/rsa_leaf-8         545 ± 0%       182 ± 0%  -66.61%  (p=0.000 n=20+20)

Fixes #21118
Fixes #44237

Change-Id: Id653f6ae5e405c3cbf0c5c48abb30aa831e30107
Reviewed-on: https://go-review.googlesource.com/c/go/+/274234


Trust: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>