Update wasm-bindgen-futures to futures-preview-0.3.0-alpha.18

[ibaryshnikov]

There was some changes in futures-preview, in particular

Changed ArcWake::into_waker to a free function waker

[alexcrichton]

Done in #1716!

[ibaryshnikov]

Cool, didn't notice it

[najamelan]

@alexcrichton Do you think it would be possible to roll out a release to crates? This breaks unit tests and compilation for anything that pulls in alpha.18...

[alexcrichton]

Sure thing, I've started that at #1722