Drop dependency on c-t-go x509 and asn1 forks

[AlCutter]

The original CTFE implementation contained forks of Go's x509 and asn1 packages, at the time this was necessary in order to make them more permissive in what they'd accept. This has become considerably less important now that tools like cablint have been adopted by the industry.

Maintaining these forks comes with a) significant work to keep them up-to-date, or b) risk that not updating them leaves them vulnerable to known issues, and/or they otherwise rot.

We should use this opportunity to switch [back] to Go's stdlib x509 and asn1 packages.