binaryen dependency feature

Created by: ascjones

In https://github.com/paritytech/cargo-contract/pull/95 we add the binaryen as a dependency, rather than requiring manual installation. However this

• Increases the build time which we want to keep as fast and easy as possible. (edit: it might not be by much? need to confirm with fresh build)
• Takes up quite a lot of disk space
• Requires the prerequisities to be installed.

We could still provide this but behind a feature, and default to using an installed binary, and perhaps we could provide better information to be able to download a prebuilt binary e.g. there is one here https://packages.debian.org/buster/binaryen.

Potentially we could detect the presence of the binary on the PATH, and then choose whether to enable this feature but it would require some trickery in build.rs so not sure about that.

/cc @Robbepop