Make library fail to compile if neither async-std nor tokio are selected with futures feature

[nazar-pc]

I just hit #16 and found it counter-intuitive.
Would have been nicer if it failed to compile with a nice error message, suggesting to choose between async-std or tokio.

[nazar-pc]

Also I don't really see on the surface why it would be required at all for basic futures support (didn't look into the code).