oiddbgen: parse types aliased to OBJECT IDENTIFIER

As part of RFC 5280, there are a few types that are essentially aliases for OBJECT IDENTIFIER.

One example is the AttributeType used in the x509 subject name:

AttributeType ::= OBJECT IDENTIFIER

-- Arc for standard naming attributes

id-at OBJECT IDENTIFIER ::= { joint-iso-ccitt(2) ds(5) 4 }

-- Naming attributes of type X520name

id-at-name                AttributeType ::= { id-at 41 }
id-at-surname             AttributeType ::= { id-at  4 }
id-at-givenName           AttributeType ::= { id-at 42 }
id-at-initials            AttributeType ::= { id-at 43 }
id-at-generationQualifier AttributeType ::= { id-at 44 }

[...]

-- Naming attributes of type X520SerialNumber

id-at-serialNumber      AttributeType ::= { id-at 5 }

Since items like the id-at-name are not directly of type OBJECT IDENTIFIER, the oiddbgen does not pick them up.

I'm not sure about the best way to handle this, but I gave it a proof-of-concept at https://github.com/simpsoneric/formats/tree/object-id-aliases

The resulting src/db/gen.rs picks up the OIDs I was interested in.

Start by reviewing the oiddbgen behavior described in the issue and the referenced proof-of-concept, especially the generated src/db/gen.rs. Confirm how aliases such as AttributeType are represented, then define completion as having the relevant aliased OBJECT IDENTIFIER values picked up by the generator.