Remove useless Tuple from chain

http://forum.dlang.org/thread/jnnkjftmfizokokssixn@forum.dlang.org

Chain was using a Tuple to hold its internal state, which is actually 100% useless. Using it is a bad idea, since Tuples tend to always be the first to get hit by bugs, and introduce non-trivial operations (assign, cc, compare), all of which would be trivial otherwise.

So simply removing it seems like the right more to me.