Add exFAT support (#150)

## Summary

* Swap to updated SDCardManager which uses SdFat
* Add exFAT support
  * Swap to using FsFile everywhere
* Use newly exposed `SdMan` macro to get to static instance of
SDCardManager
* Move a bunch of FsHelpers up to SDCardManager

lib/Epub/Epub/Section.h

@@ -12,11 +12,11 @@ class Section {
   const int spineIndex;
   GfxRenderer& renderer;
   std::string filePath;
-  File file;
+  FsFile file;
 
   void writeSectionFileHeader(int fontId, float lineCompression, bool extraParagraphSpacing, int viewportWidth,
                               int viewportHeight);
-  size_t onPageComplete(std::unique_ptr<Page> page);
+  uint32_t onPageComplete(std::unique_ptr<Page> page);
 
  public:
   int pageCount = 0;