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/Serialization/Serialization.h

@@ -1,5 +1,5 @@
 #pragma once
-#include <FS.h>
+#include <SdFat.h>
 
 #include <iostream>
 
@@ -10,7 +10,7 @@ static void writePod(std::ostream& os, const T& value) {
 }
 
 template <typename T>
-static void writePod(File& file, const T& value) {
+static void writePod(FsFile& file, const T& value) {
   file.write(reinterpret_cast<const uint8_t*>(&value), sizeof(T));
 }
 
@@ -20,7 +20,7 @@ static void readPod(std::istream& is, T& value) {
 }
 
 template <typename T>
-static void readPod(File& file, T& value) {
+static void readPod(FsFile& file, T& value) {
   file.read(reinterpret_cast<uint8_t*>(&value), sizeof(T));
 }
 
@@ -30,7 +30,7 @@ static void writeString(std::ostream& os, const std::string& s) {
   os.write(s.data(), len);
 }
 
-static void writeString(File& file, const std::string& s) {
+static void writeString(FsFile& file, const std::string& s) {
   const uint32_t len = s.size();
   writePod(file, len);
   file.write(reinterpret_cast<const uint8_t*>(s.data()), len);
@@ -43,10 +43,10 @@ static void readString(std::istream& is, std::string& s) {
   is.read(&s[0], len);
 }
 
-static void readString(File& file, std::string& s) {
+static void readString(FsFile& file, std::string& s) {
   uint32_t len;
   readPod(file, len);
   s.resize(len);
-  file.read(reinterpret_cast<uint8_t*>(&s[0]), len);
+  file.read(&s[0], len);
 }
 }  // namespace serialization