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/Xtc/Xtc/XtcParser.h

@@ -7,7 +7,7 @@
 
 #pragma once
 
-#include <SD.h>
+#include <SdFat.h>
 
 #include <functional>
 #include <memory>
@@ -80,7 +80,7 @@ class XtcParser {
   XtcError getLastError() const { return m_lastError; }
 
  private:
-  File m_file;
+  FsFile m_file;
   bool m_isOpen;
   XtcHeader m_header;
   std::vector<PageInfo> m_pageTable;