The overloading of IR emits magic \X'...' sequences that turn into HTML manual links.
But not all such IR invocations should be manual links;
those had to be written to avoid the IR macro before.
Worse, the \X'...' ending the IR causes troff to emit only a single space after a period.
Defining a new IM macro for manual references fixes both problems.
Fixes#441.
List mode was constrained to the BMP. This change introduces
the following new list mode convention, using Go string literal syntax:
Non-printing ASCII characters display as \xhh.
Non-ASCII characters in the BMP display as \uhhhh.
Characters beyond the BMP display as \Uhhhhhhhh.
