Implement OptionIntoWasmAbi for `Option<&Closure>.

[teymour-aldridge]

I think this is the correct way to do it. I'm not sure about that though (my knowledge about WebAssembly is very limited).
This should resolve #2173.

[alexcrichton]

Thanks! Can you be sure to add some tests for this as well?

[teymour-aldridge]

Sure! Could you suggest what I should test for?

[alexcrichton]

In general I'd just exercise this feature. Make sure Some and None get passed, the basic functionality still works, everything compiles, etc.

[teymour-aldridge]

Cool! (thanks)

[ctaggart]

@teymour-aldridge thanks for starting this! Any luck adding tests?

[teymour-aldridge]

Hi, I've been a bit busy recently so haven't had time to work on this much. If you'd like to add some tests, please feel free.

[ctaggart]

I added a test to tests/wasm/closures.rs, but I'm not sure how to run the tests. When I run cargo test --target wasm32-unknown-unknown, I have 141 tests pass, but it does not include the closure tests.

[ctaggart]

I see. I was missing --features nightly.

[ctaggart]

I added a basic test that you can see in #2211. Unfortunately, this does not fix it yet and I am not sure how to.