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 change provides a way to use citation keys that contain
special characters not usable with the standard citation
key syntax. Example: `@{foo_bar{x}'}` for the key `foo_bar{x}`.
Closes#6026.
The change requires adding a new parameter to the `citeKey`
parser from Text.Pandoc.Parsing [API change].
Markdown reader: recognize @{..} syntax for citatinos.
Markdown writer: use @{..} syntax for citations when needed.
Update manual with curly-brace syntax for citations.
Closes#6026.
