Remove the stride from Matrix representation

Remove mat_save and mat_load for now

Swap the order of the parameters of canvas_to_row

Move gym_fit_square() to gym.h

Move gym_root() to gym.h

Implement Shape Classifier Demo

Get rid of separate ti and to in the API of nn.h

Introduce type for Row Vectors

Introduce GYM_BACKGROUND

TODO: allow a single slot to take up several slots

Introduce a bunch of compatibility TODOs

TODO: make nn_forward signature more natural

TODO: introduce separate types for Row Vectors

Make Region word-aligned

Refactor nn_finite_diff() to return NN, just like nn_backprop()