Suggested by G. Brandon Robinson.

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.

I changed from 6 to 8 digits but forgot to update the man page.

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.