support passing nil to R509::ASN1.general_name_parser
small validation point on cdp_location
modify CertificateAuthority::Signer to support multiple GeneralName

types in cdp_location, ocsp_location, and ca_issuers location. The types
are auto-parsed using R509::ASN.general_name_parser (just like we do
in R509::CSR and other places)
modify authorityKeyIdentifier to no longer encode the issuer since
that's not a thing most "real" CAs do