bpo-40528: Implement a metadata system for ASDL Generator (GH-20193)

ASDL Generator was lack of proper annotation related to generated
module. This patch implements a MetadataVisitor that produces a
metadata object to pass to other visitors that are visiting that
same module. For the inital patch, it dynamically retrieves int
sequences (like cmpop), that was previously hardcoded. It offers
an interface that is easy to extend.