Add spec for Transducers

Hi! We've been talking about Transduce and Transducers in the Sanctuary Gitter channel and there are mixed thoughts. There's definitely curiosity and value in them since there's general interest expressed around the internet and on a variety of other mediums, so I'm placing this issue to ponder around the concept, and perhaps eventually find a suitable solution together.

Some links follow.

• For a working implementation of Transduce, we can look at Ramda's: http://ramdajs.com/docs/#transduce which has it's type signature as: (c → c) → (a,b → a) → a → [b] → a.
• For a guided trip to Transducers in JavaScript: https://github.com/getify/Functional-Light-JS/blob/master/apA.md
• For Clojure docs on Transducers: https://clojure.org/reference/transducers
• For a journey into their type definition through Haskeller's eyes: http://hypirion.com/musings/haskell-transducers
• For a great talk about Transducers at Strange Loop, by Rick Hickey: https://www.youtube.com/watch?v=6mTbuzafcII