Add \o{} escape

Add \o{} escape

This commit adds the new construct \o{} to express a character constant
by its octal ordinal value, along with ancillary tests and
documentation.

A function to handle this is added to util.c, and it is called from the
3 parsing places it could occur.  The function is a candidate for
in-lining, though I doubt that it will ever be used frequently.