Deno: Move away from excessive use of `FutureExt`

[kevinkassimo]

FutureExt provides our familiar .then, .and_then, .map etc.

However, after switching to std::future::Future, in many places we can switch directly to using async/async move block, or async function (when lifetimes of arguments are not a problem)

In practice, it might be able to save us some unnecessary clones and make the code more readable and tidy.

A demo of such refactoring is #3359

[kevinkassimo]

For new contributors who want to work on this:
Search in the codebase the keyword FutureExt, especially many files under cli/ops/.... Try replacing some places where .then/.and_then/.map etc. is used with a complete async block which returns a future.

[bartlomieju]

FYI I'm working on update to tokio 0.2 which will should remove a few of these usages.

[bartlomieju]

I think we can close this issue now, we're left with around 20 imports of FutureExt 👍