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.)
* Use a Span with class "title-reference" for the default
title-reference role.
* Use B.text to split up contents into Spaces, SoftBreaks, and Strs
for title-reference.
* Use Code with class "interpreted-text" instead of Span and Str for
unknown roles. (The RST writer has also been modified to round-trip
this properly.)
* Disallow blank lines in interpreted text.
* Backslash-escape now works in interpreted text.
* Backticks followed by alphanumerics no longer end interpreted text.
Closes#4811.
