Use the same allocation syntax in all allocation sites to make sure one
implementation class will flow into the fields.

For `PbMap`, this doesn't make any difference, but makes the intention
clear. Map calls are currently already monomorphic.

For `PbList`, the list field becomes monomorphic growable list (from the
list base class). This makes `add` calls monomorphic and inlinable, and
avoids double mutability checks (once in `PbList.add`, again in
`_wrappedList.add`).