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/EpubTocEntry.h

@@ -2,12 +2,9 @@
 
 #include <string>
 
-class EpubTocEntry {
- public:
+struct EpubTocEntry {
   std::string title;
   std::string href;
   std::string anchor;
-  int level;
-  EpubTocEntry(std::string title, std::string href, std::string anchor, const int level)
-      : title(std::move(title)), href(std::move(href)), anchor(std::move(anchor)), level(level) {}
+  uint8_t level;
 };

lib/Epub/Epub/parsers/TocNcxParser.cpp

@@ -155,7 +155,7 @@ void XMLCALL TocNcxParser::endElement(void* userData, const XML_Char* name) {
       }
 
       // Push to vector
-      self->toc.emplace_back(self->currentLabel, href, anchor, self->currentDepth);
+      self->toc.push_back({std::move(self->currentLabel), std::move(href), std::move(anchor), self->currentDepth});
 
       // Clear them so we don't re-add them if there are weird XML structures
       self->currentLabel.clear();

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);