add a `--binaryen` flag

[redianthus]

that would use binaryen to optimize a given module

[kachida]

@zapashcanon ,I’m new to the OCaml ecosystem and I’m interested in working on this task. Could you kindly assign me to this issue.

[redianthus]

Hi @kachida, yes sure, you can try to implement it. No need to assign you, I'm only assigning people at my company so we get an idea of who could do what. If one issue is interesting you, feel free to pick it up. If you need any help, you can join Owi's Zulip.