Add Cascade implementation

[donny-dont]

Adds the Cascade class and its associated tests

[donny-dont]

This is dependent on #85

Enabling tests is dependent on dart-archive/package_resolver#4

[nex3]

I'm not sure a Cascade makes sense for http. It's important for shelf as a kind of super-simple routing, but I don't see a strong enough use-case for the client to warrant making it part of the core package.

[donny-dont]

Ah okay in a previous conversation I swore you mentioned that as something to port. Your call if you want to close it as it was just a copy and paste from shelf with a couple modifications.

[nex3]

It's possible I did—I've been known to make silly mistakes on occasion 😉. In any case, I've changed my mind, so I'm gonna close this out.