documentation errors or imprecisions.

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.

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

in all .c files that needed it.

their name, eg. cucul_create() -> cucul_create_canvas() etc.
  * Moved buffer-related functions to cucul/buffer.c.