Dimension checking refactor
    * Dimensions.hs
        * Combined HasInput, HasOutput, DimMatchable into SizedOperator
        * Actually has enforcable laws (dimsMatch == True guarantees
          total function)
        * TODO: Document law
    * Switched all instances of the above to new refactoring
    * Layer.hs
        * Corrected outSize (was inSize before somehow)
    * Misc
        * Removed some extraneous imports in NeuralNet
        * Updated Nunavut exports
Activator tests added
    * Added Nunavut.Util.TestUtils
        * common Specs and helper functions
    * Added Nunavut.ActivatorSpec
        * codomain, completeness, and monoticity of all
          activators checked
        * derivative law checked for all activators
Added Filter tests
    * Newtypes.hs
        * Added HasMtx and SizedOperator instances for Jacobian
    * Filter.hs
        * Softmax bugfix: InfNorm => maxElement
    * Arbitrary
        * Added SmallMtx and SmallVec newtypes for restricting sizes
    * TestUtils
        * Added diffJacob for numerical computation of jacobian
          mtx by forward differences (SLOW. use only for testing
          and only with SmallMtx/SmallVec)
    * FilterSpec
        * Test function totality, dimension consistency
        * Test softmax L2 norm always 1
        * Test derivative law