Chaned Updates monoid, switched propogation to RWS
    * Propogation
        * Updates now a newtype
            * Monoid instance adds corresponding matrices
              instead of concatenation
        * Concatenation of each set of updates now done
          (**somewhat unsafely**) through State
        * Consequently, switched to RWS instead of explicit
          stack. Simpler types anyway.
    * Weights
        * Now modify state instead of telling the Update
    * NeuralNet
        * Packs up the current Update state and tells it to
          the Writer, clears the current state.