src/HOL/SMT/Examples/cert/z3_linarith_13

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 ( T1)
:extrafuns (
  (uf_2 T1)
  (uf_3 Int Int T1)
  (uf_1 Int Int T1)
  (uf_4 Int)
 )
:assumption (forall (?x1 Int) (?x2 Int) (iff (= (uf_1 ?x1 ?x2) uf_2) (<= ?x1 ?x2)))
:assumption (forall (?x3 Int) (?x4 Int) (iff (= (uf_3 ?x3 ?x4) uf_2) (< ?x3 ?x4)))
:assumption (not (distinct (uf_3 uf_4 3) (uf_1 3 uf_4)))
:formula true
)