The only changes here other than moving chunks of code around are
- moving `evaluate` out of `impl Script`, which required changing
 `&self` to `script: &Script`; and
- unifying `ExecutionOptions` with `VerificationFlags`.

For easier side-by-side comparison.

The C++ impl uses exceptions for `ScriptNum`, but catches them.

These tests run both the C++ and Rust impls. One uses completely
arbitrary inputs, and the other limits script_sig to data pushes.

For now, the underlying errors are discarded when comparing against the
C++ results, but there are corresponding changes on the C++ side in a
separate branch.