src/HOL/SMT/Examples/cert/z3_bv_bit_07

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_1 Int T1)
  (uf_2 T1 Int)
 )
:assumption (forall (?x1 T1) (= (uf_1 (uf_2 ?x1)) ?x1))
:assumption (forall (?x2 Int) (implies (<= 0 ?x2) (= (uf_2 (uf_1 ?x2)) ?x2)))
:assumption (forall (?x3 Int) (implies (< ?x3 0) (= (uf_2 (uf_1 ?x3)) 0)))
:assumption (not (= (extract[2:1] bv22[4]) bv3[2]))
:formula true
)