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

src/MappedInputManager.h

@@ -2,8 +2,6 @@
 
 #include <InputManager.h>
 
-#include "CrossPointSettings.h"
-
 class MappedInputManager {
  public:
   enum class Button { Back, Confirm, Left, Right, Up, Down, Power, PageBack, PageForward };