fix parsing of C++ fields

C++ fields are parsed like ordinary variable declarations, for more flexibility (storage, initial values etc). But to be transformed, they need to be converted back in to the standard representation of a structure field. This commit makes that change.

Note that the C++ parser accepts a lot more for structure fields than the SmPL parser. The SmPL parser currently only accepts the C syntax for structure fields, so no storage or initial values. Fields with this extra information can be matched with field metavariables, but not with actual patterns. That is, there is no isomorphism that ignores storage or initialization for fields.