R-mode: Support prettify-symbols-mode.

Prettify-symbols-mode is new in the upcoming Emacs release.  It can
convert symbols into unicode representation, e.g., <- to ←.  This patch
sets the `prettify-symbols-alist`.  The user can then activate the
prettify mode by calling `prettify-symbols-mode`.

* lisp/ess-r-d.el (R-mode--prettify-symbols-alist): New constant.
  (R-mode): Set `prettify-symbols-alist`.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>