add non-breaking space before em-dashes. it may not yet actually
be non-breaking in practice.
insert space following em-dashes
fix width of non-breaking spaces (0xa0)
fix insertion of non-breaking space before em-dash in various
situations, including where the em-dash immediately follows a
footnote or other tex scope closure, i.e., a }.
unify line checkpoint calculation.
update line checkpoint selection routine to ensure that
non-breaking spaces are, in fact, non-breaking.