Implement FieldDetails.equal()/hash() in terms of the default.

Implement FieldDetails.equal()/hash() in terms of the default.

This allows explicit references to FieldDetails.equal()/hash() for
structures storing them (in case it changes from the
AbstractFieldDetails implementation in the future).

Helps: bgo#653679 - Change PostalAddressDetails.postal_addresses to
support vCard-like arbitrary parameters