Factor out FA existential wrapper

This makes sense now it has been reimplemented on multiple occasions (in
VinFormat/Meta to build VinFormat macros and Contract to provide a list
of identifier fields).

To make it useful, field introspection helpers are lifted to use this
wrapper (fieldNameE, fieldTypesQ).
Fix dictionaryLabel for (IdentI Partner) fields
Attach ContractsDict to contractIdentifier f-me/carma#1303