Restore experimental code related to `x509`

[jorisdral]

#49 removes experimental code related to x509 because it does not compile any more. It would be great if we could restore this code.

This might also require us to update the experimental fork of the C Botan library at https://github.com/apotheca/botan-upstream. Moreover, if we would want to un-experimentalise these features, we would probably have to upstream the changes on the fork to the main repository.

[jorisdral]

See also the removed test code in ed412e5