Move examples / helpers to separate crates

[zimond]

This is a follow-up of #62 . I think currently most of the utility methods in utils are not need in production. They are just adding up binary / wasm file size. Why don't we move them to a separate crate and add it as a dependency in tests / examples? If you guys think this is OK, I could setup a PR about this.

[pixelspark]

@zimond sounds like a good idea - you might also move these to wonnx-preprocessing which has some other bells and whistles that are not needed by all users.