Improve handling of references.

OpenAPI descriptions with large numbers of repeated $refs can take a long time to resolve. This is because we explicitly expand all $refs.

For example, this runs much faster with --keep_refs than without it:
`openapic --sample_out=- https://api.apis.guru/v2/specs/bitbucket.org/2.0/swagger.yaml --keep_refs`

A better approach would be to keep $refs in place but still expand each unique $ref and store the expansion in the compiled protos so it could be easily used in client code when needed.