RST reader: improve parsing of inline interpreted text roles.

* 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.

1 files changed, 48 insertions, 0 deletions

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+No blank lines in inline interpreted roles:
+
+```
+% pandoc -f rst -t native
+`no
+
+blank`:myrole:
+^D
+[Para [Str "`no"]
+,Para [Str "blank`:myrole:"]]
+```
+
+Backslash escape behaves properly in interpreted roles:
+
+```
+% pandoc -f rst -t native
+`hi\ there`:sup:
+
+`hi\ there`:code:
+^D
+[Para [Superscript [Str "hithere"]]
+,Para [Code ("",["sourceCode"],[]) "hi\\ there"]]
+```
+
+Backtick followed by alphanumeric doesn't end the span:
+```
+% pandoc -f rst -t native
+`hi`there`:myrole:
+^D
+[Para [Code ("",["interpreted-text"],[("role","myrole")]) "hi`there"]]
+```
+
+Newline is okay, as long as not blank:
+```
+% pandoc -f rst -t native
+`hi
+there`:myrole:
+^D
+[Para [Code ("",["interpreted-text"],[("role","myrole")]) "hi\nthere"]]
+```
+
+Use span for title-reference:
+```
+% pandoc -f rst -t native
+`default`
+^D
+[Para [Span ("",["title-ref"],[]) [Str "default"]]]
+```