Swap out Debug.impossible_ with spec_assert_fail (or similar)

[barockobamo]

From #26 :

jsinterpreter.ml: replace
(fun s m -> Debug.impossible_with_heap_because LOC s m; Result_impossible) s ("[run_object_method] returned a primitive in [object_proto_is_prototype_of_body].")
with (after defining an « impossible » function)
impossible s "[run_object_method] returned a primitive in [object_proto_is_prototype_of_body]."

[IgnoredAmbience]

This has been partially done in places already, any remaining cases of Debug.impossible in JsInterpreter should be replaced by let%assert or spec_assert or spec_assert_fail, or similar depending on usage.