Added initial versions of replace-chars-with-strings() and

Added initial versions of replace-chars-with-strings() and
apply-character-map() functions. These are intended mainly for use
in the manpages stylesheets but may be useful elsewhere too.

I need to fix the logic in the manpages stylesheet so that the
character-map file is read only once per document. The way it is
now, the character map is read each time a refentry is found,
which is a big waste.