Previously we used our own homespun formatting. But this
produces over-long lines that aren't ideal for diffs in tests.
Easier to use something off-the-shelf and standard.
Closes#7580.
Performance is slower by about a factor of 10, but this isn't
really a problem because native isn't suitable as a serialization
format. (For serialization you should use json, because the reader
is so much faster than native.)
the token string is modified by a parser (e.g. accent when
it only takes part of a Word token).
Closes#5686. Still not ideal, because we get the whole
`\t0BAR` and not just `\t0` as a raw latex inline command.
But I'm willing to let this be an edge case, since you
can easily work around this by inserting a space, braces,
or raw attribute. The important thing is that we no longer
drop the rest of the document after a raw latex inline
command that gobbles only part of a Word token!
