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/ZipFile/ZipFile.h

@@ -1,19 +1,19 @@
 #pragma once
 #include <Print.h>
 
-#include <functional>
 #include <string>
 
 #include "miniz.h"
 
 class ZipFile {
   std::string filePath;
+  mutable mz_zip_archive zipArchive = {};
   bool loadFileStat(const char* filename, mz_zip_archive_file_stat* fileStat) const;
   long getDataOffset(const mz_zip_archive_file_stat& fileStat) const;
 
  public:
-  explicit ZipFile(std::string filePath) : filePath(std::move(filePath)) {}
-  ~ZipFile() = default;
+  explicit ZipFile(std::string filePath);
+  ~ZipFile() { mz_zip_reader_end(&zipArchive); }
   bool getInflatedFileSize(const char* filename, size_t* size) const;
   uint8_t* readFileToMemory(const char* filename, size_t* size = nullptr, bool trailingNullByte = false) const;
   bool readFileToStream(const char* filename, Print& out, size_t chunkSize) const;