Support for user-provided runtime about async-scoped

[ftelnov]

@rmanoka Please, take a look at #16 for an implementation and review

[rmanoka]

Nice work @ftelnov .

I think I'd made the trait private, to ensure we're not violating any rust memory invariants. In particular, I wasn't sure if there's a safe-rust executor, whose guarantees may not match the requirements of this crate, and hence implementing the traits on this executor leads to memory-UB even via safe-rust.

As an escape hatch, I'm okay with making the trait unsafe, and explaining that the user has to prove/verify the necessary things before exposing it via safe-rust.

[ftelnov]

At the first glance at feature gates I realized you actually did it on purpose. Thanks for the explanation!
I think we simply can mark the traits unsafe, because it solves the problem and delegates responsibility of necessary checks to the end-user. I also think it's okay from the perspective of unsafe-code pervasivity - in the end, those traits are not being implemented often.

I've added unsafe markers and write yet simple top-level comment about traits unsafety.

But what should be additionally done is invariants definition. What contracts must implementer of these unsafe traits uphold? In other words, what are requirements for the executor? Do we have an example of the dangerous executor?

[rmanoka]

Thanks @ftelnov ; have merged this. I'll release it in the next couple weeks.

[rmanoka]

Released as 0.8.0