runtime: Support async-std

[akiradeveloper]

Aside one for tokio, better to have a runtime support for async-std.

[akiradeveloper]

Another strategy is runtime-agnostic way like hyper does if we can share the rest of the code but spawning.

Agnostik is a library for this purpose but it unfortunately seems unmaintained.

Good articles about runtime agnostic:

• https://rust-lang.github.io/wg-async/vision/submitted_stories/status_quo/barbara_writes_a_runtime_agnostic_lib.html
• https://www.reddit.com/r/rust/comments/i5hppj/smol_vs_tokio_vs_asyncstd/
• writing a library that can be reused across many runtimes rust-lang/wg-async#45

[akiradeveloper]

https://www.reddit.com/r/rust/comments/fo98kc/announcing_agnostik_executor_agnostic_runtime/

I learned there is an alternative of agnostik called async_executors. This one looks more maintained and supports more runtimes including glommio. (blog: https://blog.wnut.pw/2020/02/25/anouncing-async_executors-a-building-block-for-executor-agnostic-libraries/)

This one looks fascinating to me.

[akiradeveloper]

In async-executors, the API is not provided to use the current global Tokio runtime.

The discussion is here

[akiradeveloper]

Unlike tokio one, JoinHandle in async-executors doesn't have abort method but we could use Abortable to cancel the request.

[akiradeveloper]

Experimental branch is here https://github.com/akiradeveloper/norpc/tree/generic-runtime

[akiradeveloper]

done.