Async port scanner

[mre]

What?

Write an async port scanner.

Why?

No Rust port scanner exists so far that is async.
Good, simple use-case to introduce the (now stable) async ecosystem.

How?

Could be fun to use an existing port scanner to build upon.
Here are two simple ones:

• https://github.com/mylamour/rscan/
• https://github.com/mdlayher/scanrs/