Implement OptionIntoWasmAbi for Closure references

[sinking-point]

See #2767

[alexcrichton]

Can you add some tests where None is used? Otherwise I think it would be useful to add a test or double-check that use of the closure from JS after the function has returned throws a well-defined exception

[sinking-point]

@alexcrichton Thanks for your feedback.

I meant to uncomment the None test before but it seems I forgot, so I've done that now and tightened it a bit too. Since there are no variations on None, I think this one test is sufficient.

Unlike Fn, FnMut and FnOnce, Closures are not invalidated after the imported function returns. I have instead added a test for the behaviour after .drop()ping the Closure. I manually checked the message of the exception:

Error: closure invoked recursively or destroyed already

This is as expected.

[alexcrichton]

Ah yes, my mistake! Thanks!

[sinking-point]

No worries. Thanks for merging.