Keep ZipFile open to speed up getting file stats. (#76)

Still a bit raw, but gets the time required to determine the size of
each chapter (for reading progress) down from ~25ms to 0-1ms.

This is done by keeping the zipArchive open (so simple ;)).

Probably we don't need to cache the spine sizes anymore then...

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Co-authored-by: Dave Allie <dave@daveallie.com>

lib/Epub/Epub.h

@@ -33,6 +33,7 @@ class Epub {
   bool parseContentOpf(const std::string& contentOpfFilePath);
   bool parseTocNcxFile();
   void initializeSpineItemSizes();
+  static bool getItemSize(const ZipFile& zip, const std::string& itemHref, size_t* size);
 
  public:
   explicit Epub(std::string filepath, const std::string& cacheDir) : filepath(std::move(filepath)) {