(diff-mode-*-map): use `easy-mmode-defmap'.

(diff-mode-*-map): use `easy-mmode-defmap'.
(diff-end-of-hunk): Return the end position for use in
`easy-mmode-define-navigation'.
(diff-recenter): Remove.
(diff-(next|prev)-*): Rename `diff-*-(prev|next)' and defined in terms
of `easy-mmode-define-navigation'.
(diff-kill-*): Rename `diff-*-kill' (for consistency with the
previous renaming) and fix to use new names.
(diff-merge-strings): Use \n as separator: simpler, faster.
(diff-mode): Use `define-derived-mode'.