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.cpp

@@ -1,5 +1,7 @@
 #include "MappedInputManager.h"
 
+#include "CrossPointSettings.h"
+
 decltype(InputManager::BTN_BACK) MappedInputManager::mapButton(const Button button) const {
   const auto frontLayout = static_cast<CrossPointSettings::FRONT_BUTTON_LAYOUT>(SETTINGS.frontButtonLayout);
   const auto sideLayout = static_cast<CrossPointSettings::SIDE_BUTTON_LAYOUT>(SETTINGS.sideButtonLayout);