src/HOL/SMT/Examples/cert/z3_pair_01

added proof reconstructon for Z3,
added certificates for simpler re-checking of proofs (no need to invoke external solvers),
added examples and certificates for all examples,
removed Unsynchronized.ref (in smt_normalize.ML)

(benchmark Isabelle
:extrasorts ( T2 T3 T1)
:extrafuns (
  (uf_2 T1 T2)
  (uf_3 T1 T3)
  (uf_1 T2 T3 T1)
  (uf_6 T2)
  (uf_4 T2)
  (uf_5 T3)
 )
:assumption (forall (?x1 T1) (= (uf_1 (uf_2 ?x1) (uf_3 ?x1)) ?x1))
:assumption (forall (?x2 T2) (?x3 T3) (= (uf_3 (uf_1 ?x2 ?x3)) ?x3))
:assumption (forall (?x4 T2) (?x5 T3) (= (uf_2 (uf_1 ?x4 ?x5)) ?x4))
:assumption (= (uf_2 (uf_1 uf_4 uf_5)) uf_6)
:assumption (not (= uf_4 uf_6))
:formula true
)