booksniff: RTF, Microsoft Reader, FictionBook and comic archives

Four formats a library of ebooks holds and this had no name for, so each
came back as text, as HTML or as unknown.

RTF is its opening group, after any byte-order mark and blank space.
Microsoft Reader is its signature at the front. A FictionBook is XML
whose root element is FictionBook, found by stepping over the
declaration, comments and doctype in the first four kilobytes. That walk
is over bytes rather than text, because a great many FictionBooks are
windows-1251 and would otherwise be called binary; other XML is still
HTML, as before.

A zip whose only real member is one FictionBook is zip-wrapped-fb2, the
same way a zipped PDF is zip-wrapped-pdf: handed to a converter as
FictionBook it is still a zip. A zip whose every real member is a JPEG,
PNG, GIF or WebP is cbz, once directories, ComicInfo.xml and desktop
clutter are passed over. One PDF or one readme among the pages and it is
not a comic. Only the names in the archive's directory are read, as for
every other zip.

The kind vocabulary grows by five: lit, fb2, rtf, cbz and
zip-wrapped-fb2.

crates/corpus-core/src/doc.rs

@@ -77,6 +77,12 @@ pub enum Kind {
     Azw3,
     /// Amazon MOBI.
     Mobi,
+    /// Microsoft Reader.
+    Lit,
+    /// FictionBook, the XML ebook format.
+    Fb2,
+    /// Rich Text Format.
+    Rtf,
     /// Plain text.
     Txt,
     /// HTML or XHTML.
@@ -85,6 +91,10 @@ pub enum Kind {
     Ole2,
     /// A zip archive whose only real content is a single PDF.
     ZipWrappedPdf,
+    /// A zip archive whose only real content is a single FictionBook.
+    ZipWrappedFb2,
+    /// A comic book archive: a zip of page images.
+    Cbz,
     /// Recognised as none of the above.
     Unknown,
 }
@@ -118,6 +128,11 @@ mod tests {
             (Kind::Ole2, "\"ole2\""),
             (Kind::Azw3, "\"azw3\""),
             (Kind::ZipWrappedPdf, "\"zip-wrapped-pdf\""),
+            (Kind::ZipWrappedFb2, "\"zip-wrapped-fb2\""),
+            (Kind::Fb2, "\"fb2\""),
+            (Kind::Lit, "\"lit\""),
+            (Kind::Rtf, "\"rtf\""),
+            (Kind::Cbz, "\"cbz\""),
             (Kind::Unknown, "\"unknown\""),
         ];
         for (kind, wire) in cases {