Support for PSR-17 about mezzio

[weierophinney]

The current status quo exists because our factories pre-dated PSR-17; I'd love to have standard PSR-17 factories instead. If we can coordinate the other repos to use PSR-17 first, and to have the factories we provide proxy to PSR-17 factories, I think we can likely make this work pretty seamlessly, and potentially without a major version required.

[boesing]

We could avoid having a major bump when creating the anonymous callable within this package.

The major bump could get rid of the callable and instead passing the PSR-17 factories instead.

I'll see if I can make a PoC next week.