add Certificate.from_pem and Certificate.from_der

[Aurvandill]

This adds the two requested functions from #274.

[djc]

I don't think this makes sense. What are you trying to do that the 0.14.x API does not let you do today?

[Aurvandill]

I'm trying to read in a certificate which is pem encoded to sign something later on. I could use an Issuer::from_ca_cert_pem to load the cert as well but i wan't to display the cert for confirmation before loading the key.
Also since the mentioned issue was still open I guessed it was still needed.

[Aurvandill]

It would have also been nice to be able to do something like

let cert = Certificate::from_pem(pem_bytes);
let (_, x509_cert) = X509Certificate::from_der(certificate.der())?;
println!("{:?}", x509_cert.tbs_certificate.validity)

Or is there perhaps another way to get CertificateParams from a pem file?

[djc]

Since 0.14, we no longer support getting a CertificateParams from DER or PEM, because we want to avoid API that allows round-tripping certificates -- potentially with surprising results. If there are things you want to display, suggest parsing it separately (we recommend using the x509-parser crate for this).

[Aurvandill]

thanks for the info.