Better interoperability with x509-parser

[matejcik]

It should be possible to generate CertificateParams from an X509Certificate directly, instead of having to go through DER decode/encode. Also methods like DistinguishedName::from_name are private while they could presumably be public.

[matejcik]

instead of having to go through DER decode/encode

ah, even worse, x509-parser can't actually serialize, so if I throw away the incoming DER representation, I'm hosed :(

[est31]

Right now, I think there is zero dependence in the public API on x509-parser types, in other words, we could theoretically replace that crate completely and it wouldn't break user code (outside of potential behaviour changes). Improving interop would prevent that flexibility. Also, a semver breaking upgrade of x509-parser would now be one for rcgen, too.