Async runtime agnostic Host Client

[Bluefinger]

It would be nice to abstract some of the client handling to not be runtime dependent, as I have a project that would greatly benefit from something like this. However, it already uses a different async runtime so I do not really want to have to run multiple runtimes just to have things work.

From what I can see, we could probably swap out usage of select! for something like futures-concurrency, which provides different approach to doing select code without the use of macros (and is runtime agnostic). Everything else will probably need abstractions to wrap around the runtimes, similar to how hyper approached this. You could do a default impl of tokio, but then leave it to others to implement the traits for their runtimes. Else, if not on the cards, I will have to yak shave something xD

[jamesmunns]

I'm open to a PR that allows for a separate runtime.

I don't believe tokio's select! is runtime dependent, but I suppose you'll still need to spend time building tokio to just get the macro. You're also likely to run into our use of tokio channels, but those are also not runtime dependent afaik.

It would be a breaking change, but I would prefer an approach similar to what ergot did in jamesmunns/ergot#93, split the std feature into std and tokio-std, and whatever other RUNTIME-std feature you'd like to introduce.

Alternately, I'm open to any PRs that allow for "manual" construction of a HostClient, allowing you to spawn your own workers.

We're not particularly dependent on any of tokio's features, we mostly use a few non-runtime specific methods, and the ability to spawn the necessary worker tasks.