Better reccursive function handling

• Use new define-fun-rec in order to instantiate more rapidly
• unrelated, last lasteffort instanciation are useful and their priority bumped

src_colibri2/tests/solve/smt_nra/unsat/dune.inc

@@ -28,3 +28,6 @@
 (rule (alias runtest) (action (run %{bin:colibri2} --size=100M --time=30s --max-steps 3500 --check-status unsat 
 --dont-print-result %{dep:mul_pos_zero_le.smt2})))
 (rule (alias runtest-learning) (action (run %{bin:colibri2} --size=100M --time=30s --max-steps 3500 --check-status unsat --learning --dont-print-result %{dep:mul_pos_zero_le.smt2})))
+(rule (alias runtest) (action (run %{bin:colibri2} --size=100M --time=30s --max-steps 3500 --check-status unsat 
+--dont-print-result %{dep:mult_hard.smt2})))
+(rule (alias runtest-learning) (action (run %{bin:colibri2} --size=100M --time=30s --max-steps 3500 --check-status unsat --learning --dont-print-result %{dep:mult_hard.smt2})))