Get (decoded) extension value

[voltone]

Right now there are high-level APIs for creating extensions (Certificates, and soon CRLs and CRL entries), but when looking up extensions an :Extension record is returned. Complicating things, the extensions returned by the X509.Certificate module from the :OTPCertificate record have their values DER decoded, while CRL extensions are returned with DER binary values, even for the same extension type (e.g. AKI).

Need to define a unified API for working with extension values.