built-in font so that we now cover all of CP437.

background.

it will eventually evolve into something better.

a file and a memory area into a libcucul buffer.
  * Changed the cucul_import_canvas() prototype so that it uses libcucul
    buffers instead of simple memory areas.

sequences longer than 100 characters.
  * Documented the escape sequence format.

intermediate bytes.

* Started changing function prototypes so that they return an integer
    instead of void, just in case they might fail.

a nicer cross compilation check.