* lisp/play/5x5.el: I/ Add an arithmetic solver to suggest positions to

* lisp/play/5x5.el: I/ Add an arithmetic solver to suggest positions to
click on. II/ Make 5x5 multisession. III/ Ensure that random grids
always have a solution in grid size = 5 cases.
(5x5-mode-map): Add keybinding to function `5x5-solve-suggest'.
(5x5-solver-output, 5x5-log-buffer): New vars.
(5x5-grid, 5x5-x-pos, 5x5-y-pos, 5x5-moves, 5x5-cracking):
Make these variables buffer local to achieve 5x5 multi-session-ness.
(5x5): Set 5x5-grid-size only if SIZE is non-negative.
(5x5-grid-to-vec, 5x5-vec-to-grid, 5x5-log-init, 5x5-log, 5x5-solver)
(5x5-solve-suggest): New funs.
(5x5-randomize): Use 5x5-make-move instead of 5x5-flip-cell to
randomize a grid so that we ensure that there is always a solution.
(5x5-make-random-grid): Allow other movement than flipping.