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.)
* These were added by the RST reader and, for literate Haskell,
by the Markdown and LaTeX readers. There is no point to
this class, and it is not applied consistently by all readers.
See #5047.
* Reverse order of `literate` and `haskell` classes on code blocks
when parsing literate Haskell. Better if `haskell` comes first.
