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
:assumption (not (not (exists (?x1 Int) (?x2 Int) (?x3 Int) (= (+ (* 4 ?x1) (* (~ 6) ?x2)) 1))))
:formula true
)