Users should be able to implement their own executors

[Stupremee]

Currently, users are not able to implement the AgnostikExecutor trait for their own executors, because it's impossible to create an instance of the JoinHandle struct.

There should be a way to create a JoinHandle from a Future, which represents the Joinhandle.

[najamelan]

I would be interested in this for existing supported executors. Basically I would like to implement the traits for the wrappers in async_executors. The way I saw this was by providing std::convert::From implementations to create JoinHandle from the tokio and async-std JoinHandle types. Would you accept a PR adding those?

It doesn't address the exact issue described here for other executors though.

The reason is that async_executors has a SpawnHandle<Out> trait which is object safe, but which requires listing the output of the joinhandle as a generic. There is user demand for a non-object safe trait that doesn't have generic, but I'd rather not re-introduce a second set of these traits as agnostik already has them, and it would also improve interop.

[Stupremee]

I definitely need to implement this. I don't know why I've never implemented that, but I will try to get back to maintaining this crate in general in the next few weeks.

This will be my first item todo! 👍