Use of async_std::task::sleep instead of tokio::time::sleep in examples

[ASR-ASU]

Proposal for #2908 issue. Replacing tokio allows the following examples to run on wasm platforms:

• backgrounded_futures
• clock
• control_focus
• eval
• future
• signals
• streams

What about the documentation?

[ealmloff]

Thank you! I switched to just always using web_time in the clock example. It will fall back to std on any non-wasm platform

[ealmloff]

What about the documentation?

We should modify the documentation in the same way. If the code snippets we use can be cross platform, that would be better