Add Future to Async category

[Grinnz]

Future is a great eventloop-independent tool for async programming.

[simcop2387]

I'd also say once it's more mature and trusted by developers and it's author, Future::AsyncAwait definitely belongs here too.

[zakame]

Future plus IO::Async indeed! @leonerd++

[leonerd]

I'd hold off on the IO::Async part quite yet; I'm still working on Future::IO which might end up being a better abstraction API for "small modules" - the sorts of things that want to mostly just be self-contained synchronous stuff but leave open the option of being async and concurrent with other stuff if the end-user wishes, but the module author isn't forced into.

[karenetheridge]

fixed with #72.