src/HOL/SMT/Examples/cert/z3_linarith_15

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
:extrafuns (
  (uf_2 Int)
  (uf_1 Int)
  (uf_3 Int)
 )
:assumption (not (or (and (< uf_1 uf_2) (< uf_2 uf_3)) (or (and (< uf_1 uf_2) (= uf_2 uf_3)) (or (and (< uf_1 uf_3) (< uf_3 uf_2)) (or (and (= uf_1 uf_3) (< uf_3 uf_2)) (or (and (= uf_1 uf_2) (< uf_2 uf_3)) (or (and (< uf_3 uf_2) (< uf_2 uf_1)) (or (and (< uf_3 uf_2) (= uf_2 uf_1)) (or (and (< uf_3 uf_1) (< uf_1 uf_2)) (or (and (= uf_3 uf_1) (< uf_1 uf_2)) (or (and (= uf_3 uf_2) (< uf_2 uf_1)) (or (and (< uf_2 uf_1) (< uf_1 uf_3)) (or (and (< uf_2 uf_1) (= uf_3 uf_1)) (or (and (< uf_2 uf_3) (< uf_3 uf_1)) (or (and (= uf_2 uf_1) (< uf_1 uf_3)) (or (and (= uf_2 uf_3) (< uf_3 uf_1)) (and (= uf_3 uf_2) (= uf_2 uf_1))))))))))))))))))
:formula true
)