Make this crate `no_std`

[notgull]

If this crate were no_std, it would allow crates like async-lock and async-channel to be used on no_std as well. I envision an API with a default std feature, and without it this crate would not depend on parking (and the wait API would disappear as well).

However, the current version of the code uses a Mutex to lock the linked list containing event listeners. Theoretically, we could replace this with an atomic linked list similar to the one used in concurrent-queue.

This would help smol be able to be used in embedded systems. Any thoughts on this?

[taiki-e]

If this can be implemented without spinlock, I tend to accept this.

[notgull]

Resolved by #34