[trans] Convert Why3 terms to NIER_simple.

src/proof_strategy.ml

src/proof_strategy.mli

src/transformations/native_nn_prover.ml

src/transformations/native_nn_prover.mli

src/transformations/relop.ml

src/transformations/symbols.ml

src/transformations/symbols.mli

src/transformations/utils.ml

src/transformations/utils.mli

src/verification.ml

tests/acasxu.t