Converted from Furnace to Chain

This pull request converts Furnace to Chain, representing a fundamentally different approach to dispatching middleware.
Chain is now Middleware itself, meaning it can be linked seamlessly onto an Iron instance.

This is a _breaking change_ as `smelt` has been deprecated in favor of `link`, and `forge` is now replaced with `dispatch`.
Hmm.. I don't see an appropriate queue in std (or other crates). I suppose this works.

@reem Can you at least speak to this to confirm?