(artist-mode-init): Added comment on the setting up of the

(artist-mode-init): Added comment on the setting up of the
`artist-replacement-table' array.
(artist-get-replacement-char): New defsubst.
(artist-get-char-at-xy-conv, artist-replace-char)
(artist-replace-chars, artist-replace-string): Use it instead of
accessing `artist-replacement-table' directly.
Reported by Rubén Berenguel <ruben@maia.ub.es>.

(artist-mt): Fixed structures for cut and copy operations.