Field : make `name` the canonical String name of a field

Add `displayName` for the overridable name (can be set via annotation)
that is displayed in editors.  Relegate `declaredName` to `FieldMirror`.

As we move away from mirrors we need the canonical `name` of a Field to
be a string, so that it may be serialized as part of a data definition,
etc...  This is based on integration experience using this new branch of
fields in my application.