nlohmann::ordered_map uses a different comparison function than the one
provided via template parameter.
* Introduce a type trait to detect if object_t has a key_compare member.
* Rename object_comparator_t to default_object_comparator_t.
* Add object_comparator_t to be conditionally defined as
object_t::key_compare, if available, or default_object_comparator_t
otherwise.
* Update the documentation accordingly.
Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>
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