We changed t_map_simp, track_values and the eval_match transformation
in order to prevent them from removing terms whose head has label
 keep_on_simp. Note that simplification inside those terms is
still possible.

* src/core/term.ml
(t_map_simp): Adding the case when f has label keep_on_simp.

* src/transform/eval_match.ml
(eval_match): Adding keep_on_simp as a stop label.

* src/whyml/mlw_wp.ml
(track_values): Stopping on keep_on_simp label.