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.)
...they should only be recognized in siunitx contexts.
For example, `\l` outside of an siunitx context should be l-slash,
not l (for liter)!
Closes#4842.
