This change moves a number of common PKIX structures into
crypto/x509/pkix, from where x509, and ocsp can reference
them, saving duplication. It also removes x509/crl and merges it into
x509 and x509/pkix.

x509 is changed to take advantage of the big.Int support that now
exists in asn1. Because of this, the public/private key pair in
http/httptest/server.go had to be updated because it was serialised
with an old version of the code that didn't zero pad ASN.1 INTEGERs.

R=bradfitz, rsc
CC=golang-dev
https://golang.org/cl/4532115