Update tests.
Reason: it turns out that the native output generated by
pretty-simple isn't always readable by the native reader.
According to https://github.com/cdepillabout/pretty-simple/issues/99
it is not a design goal of the library that the rendered values
be readable using 'read'. This makes it unsuitable for our
purposes.
pretty-show is a bit slower and it uses 4-space indents
(non-configurable), but it doesn't have this serious drawback.
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 reader now parses the contents of the markdown cell to a Pandoc
structure, but *also* stores the raw markdown in a `source`
attribute on the cell Div. When we convert back to markdown,
this attribute is stripped off and the original source is used.
When we convert to other formats, the attribute is usually
ignored (though it will come through in HTML as a `data-source`
attribute, not unhelpfully).
I'll note some potential drawbacks of this approach:
- It makes it impossible to use pandoc to clean up or
change the contents of markdown cells, e.g.
going from `+smart` to `-smart`.
- There may be formats where the addition of the `source`
attribute is problematic. I can't think of any, though.
Closes#5408.
