crypto: reorg, cleanup and add function for generating CRLs.

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

tls.go

@@ -159,7 +159,7 @@ func X509KeyPair(certPEMBlock, keyPEMBlock []byte) (cert Certificate, err os.Err
 
 	key, err := x509.ParsePKCS1PrivateKey(keyDERBlock.Bytes)
 	if err != nil {
-		err = os.ErrorString("crypto/tls: failed to parse key")
+		err = os.ErrorString("crypto/tls: failed to parse key: " + err.String())
 		return
 	}