Start migration of S.S.C.X509Certificates to new Asn.1 APIs
* Read/write X509 extensions using the new ASN classes.
* Change to accept BER, because netfx accepted it.
* Migrate AppleCertificatePal's CertificateData to new ASN classes.
* Migrate CertificateRequest to new ASN classes.
* Update PKCSdotnet/corefx#9 and PKCSdotnet/corefx#10 serialization to use new ASN.1 classes.
* Add test for normalizing zero-only serial number.
* Reimplement RSAPssX509SignatureGenerator using AsnSerializer.
* Reuse PssParamAsn from S.S.C.Pkcs.
Commit migrated from https://github.com/dotnet/corefx/commit/
9853efb2db2063621a40612dc66adc06188b8778