Only a little bit of generalized logic for DER encoding is needed and so
it can live inside the Certificate namespace.
This commit removes the generic "parse object id" function which is no
longer used in favor of more specific, smaller sets of object ids used
with ComptimeStringMap.
When scanning the file system for root certificates, expired
certificates are skipped and therefore not used for verification in TLS
sessions. There is only this one check, however, so a long-running
server will need to periodically rescan for a new Certificate.Bundle
and strategically start using it for new sessions. In this commit I made
the judgement call that applications would like to opt-in to root
certificate rescanning at a point in time that makes sense for that
application, as opposed to having the system clock potentially start
causing connections to fail.
Certificate verification checks the subject only, as opposed to both the
subject and the issuer. The idea is that the trust chain analysis will
always check the subject, leading to every certificate in the chain's
validity being checked exactly once, with the root certificate's
validity checked upon scanning.
Furthermore, this commit adjusts the scanning logic to fully parse
certificates, even though only the subject is technically needed. This
allows relying on parsing to succeed later on.
