hinomaro working
Hinumaru: better text, simplified solution-extraction.
Hinomaru: more better text.
Hinomaru: cleaned up a bit.

modified:   Hinomaru_Puzzle.ipynb
    Removed tests.
    Added inverted indexes SQUARE_PLACES, FACE_PLACES, FACE_TILES.
    Did more things with those indexes rather than loops with if's.
    Simplified the setup of the constraints a bit--
    at least turned more complicated things into comprehensions.
    Got rid of face_shown_vars global, and created shown_vars out of
    tile_shows_face with a comprehension in init_shows_eq_places().
    More work on the text.