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Add negative test cases where a warning should be issued (and the generated code is rejected by OCaml).

VisitorsAnalysis: use [VisitorsCompatibility.object_field_to_core_type] for compatibility with OCaml 4.06.0.

VisitorsCompatibility: new types [str] and [object_field]; new function [object_field_to_core_type].

This should make no observable difference, as this function is used only
to produce an error message in a corner case.