Add `no_std` and `no_alloc` async runtime

[SeaRoll]

Heya, I wonder if this is allowed to be entered into the list?

It's a no_std and no_alloc async runtime.

[BartMassey]

Would be fine to add, I think: make a PR to add it and we'll review it. I think you'll want to compare with Embassy, though?

[burrbull]

Do you know any real open source no_std project using these crates? It would be fine to add link in README as example.