feat(pool): import client::pool from hyper

[dswij]

I'm giving hyperium/hyper#2860 a go, but I'm not sure I'm going in the right direction.

Appreciate if you could take a look when you have time @seanmonstar

---

Some questions:

Some part of pool uses hyper::Errors

error[E0624]: associated function `new_canceled` is private
   --> src/client/pool.rs:592:41
    |
592 | ...                   HyperError::new_canceled().with(CheckoutIsClosedError)
    |                                   ^^^^^^^^^^^^ private associated function
    |
   :::
    |
247 |     pub(super) fn new_canceled() -> Error {
    |     ------------------------------------- private associated function defined here

I'd imagine that it's probably best to expose these functions from hyper::Error, but I'm not sure that this would follow the plan for hyper-util?

[seanmonstar]

Oh right... Hm, I think this is something we'll need to experiment with, and won't have the full solution when merging this PR, and that's OK. We might just return a hyper::Error created with a Code::CANCELED, but maybe not. It could be that we want a client::pool::Error type that is separate.

It might just be easier to start with returning a Box<dyn std::error::Error + Send + Sync>, and we can change it later once it's been decided.

[dswij]

I've replaced the hyper::Errors with GenericError for now.

[seanmonstar]

Suggested change

	#[cfg(test)]
	#[cfg(all(test, not(miri)))]

Looks like Miri is grumpy with some of the tests in here (sigh). This should make it stop :)

[dswij]

Done. b162476

[seanmonstar]

Thanks!