Implement bidi-sensitive word movement with arrow keys.

Implement bidi-sensitive word movement with arrow keys.

 lisp/subr.el (right-arrow-command, left-arrow-command): Move to bindings.el.
 lisp/bindings.el (right-char, left-char): Move from subr.el and
 rename from right-arrow-command and left-arrow-command.
 (right-word, left-word): New functions.
 (global-map) <right>: Bind to right-char.
 (global-map) <left>: Bind to left-char.
 (global-map) <C-right>: Bind to right-word.
 (global-map) <C-left>: Bind to left-word.

 doc/emacs/basic.texi (Moving Point): Update due to renaming of commands bound
 to arrows.  Document bidi-aware behavior of C-<right> and C-<left>.