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.)
Also foreigncblockquote, hyphenblockquote, hyphencblockquote.
Closes#4848. But note: currently foreignquote will be
parsed as a regular Quoted inline (not using the quotes
appropriate to the foreign language).
