Use faces not variables for font-lock customization in rst.el.

Use faces not variables for font-lock customization in rst.el.

* textmodes/rst.el: (rst-block, rst-external, rst-definition, rst-directive)
(rst-comment, rst-emphasis1, rst-emphasis2, rst-literal, rst-reference):
New faces.
(rst-block-face, rst-external-face, rst-definition-face)
(rst-directive-face, rst-comment-face, rst-emphasis1-face)
(rst-emphasis2-face, rst-literal-face, rst-reference-face):
Make obsolete.
(rst-font-lock-keywords-function): Update for above changes.