Move send_future support into Yewtil

[hgzimmerman]

Because ComponentLink is now Cloneable, support for futures can be moved out of Yew proper and into this crate instead.

Additionally, this would be a good opportunity to support send_future_batch as well.

This is based on discussion within the following issue:
yewstack/yew#742.

[hgzimmerman]

I think this can be implemented as a trait used to extend the available functions on ComponentLink. This has the drawback of users having to import the trait to use it, but I think that is a fair trade-off.