Extract EPUB TOC into temp file before parsing (#85)

## Summary

* Extract EPUB TOC into temp file before parsing
* Streaming ZIP -> XML parser uses up a lot of memory as we're
allocating inflation buffers while also holding a few copies of the
buffer in different forms
* Instead, but streaming the inflated file down to the SD card (like we
do for HTML parsing, we can lower memory usage)

## Additional Context

* This should help with
https://github.com/daveallie/crosspoint-reader/issues/60 and
https://github.com/daveallie/crosspoint-reader/issues/10. It won't
remove those class of issues completely, but will allow for many more
books to be opened.

lib/Epub/Epub/parsers/TocNcxParser.h

@@ -17,7 +17,7 @@ class TocNcxParser final : public Print {
 
   std::string currentLabel;
   std::string currentSrc;
-  size_t currentDepth = 0;
+  uint8_t currentDepth = 0;
 
   static void startElement(void* userData, const XML_Char* name, const XML_Char** atts);
   static void characterData(void* userData, const XML_Char* s, int len);