[WIP] Update to Hyper 0.13

[dbcfd]

Update to hyper 0.13 and std::future::Future.

Meant for OpenAPITools/openapi-generator#4210

Currently blocked on another release of hyper, where it is easier to tower services, and make service is no longer needed.

Existing release of hyper runs into multiple implementation issues due to blanket implementations for tower::Service.

[richardwhiuk]

I'd definitely wait till hyper 0.13 is out of beta to merge this.

Apart from that, and I haven't done a detailed review, it looks good.

I'm not sure we need to block it behind tower::Service being pulled in - we may want to take that in the future, but I think that's sufficiently far off that we can do it in a later release.

[dbcfd]

I've temporarily forked just the context part of this library to continue moving forward on openapi-generator.

The changes to hyper are pretty significant (e.g. no more make service, different types), for changes I can't test very easily.

[dbcfd]

I now have openapi generator using https://github.com/dbcfd/openapi-context which uses hyper 0.13. It does not include all of this library (e.g. composites), and I'm not sure if they should be used going forward, since they're not used in openapi generator.