remove send_future from ComponentLink

[hgzimmerman]

Closes #792

This also removes the futures example.

Here is where the futures example is migrating to: yewstack/yewtil#37
And send_future now exists here: https://github.com/yewstack/yewtil/blob/596844663be16162f0bbd2f98d36f41b70096adf/src/future.rs#L7

[jstarry]

I think we should keep the example here too. It's not necessary to use yewtil to use futures

[hgzimmerman]

Understood. I'll reinstate the example and provide an implementation of send_future within it.

There is the question of #776 though. I think in the meantime, I'll implement it in yewtil and copy the stdweb-compatible implementation of it to the futures example here later?

[jstarry]

Thanks! That's just what I had in mind.

RE #776, sounds good!