feat: provide send-wrapper to contidionally implement Send for operators

[waynexia]

Closes #4437

Quote from #4437:

wasm32-unknown-unknown only has one worker thread, it's wise to use !Sync structure under that precondition. But since this marker trait would affect others, it's sometimes hard to use them in a "normal" codebase (i.e., write for multi-thread but run in wasm32-unknown-unknown).

[Xuanwo]

Thank you for the PR. I'm wondering if we have more maintainable implementations.

For instance, let's define a trait named IntoSend with a method into_send() and implement it for futures. We could then wrap the input using send_wrapper, effectively making it a no-op on other targets.

What do you think?

[waynexia]

Sounds great, let me have a try 👍

[waynexia]

Please take another look @Xuanwo

I use cargo check --target wasm32-unknown-unknown --features send_wrapper to verify if it works. Do you think we need to add this in CI?

[Xuanwo]

Thanks!

[Xuanwo]

I use cargo check --target wasm32-unknown-unknown --features send_wrapper to verify if it works. Do you think we need to add this in CI?

Feel free to do this in a new PR!