Refactor DSi_NAND (#1844)

* Refactor diskio's contents

- Change ff_disk_read_cb/write_cb into a std::function instead of a raw pointer
- Add const specifiers as needed

* Refactor DSi_NAND to manage the file system's mounted lifetime with RAII

* Split NANDMount into NANDMount and NANDImage

- NANDImage is used for information about the NAND that doesn't require decryption or filesystem access
- NANDMount is used to actually access the file system
- Both classes manage their respective resources (the NAND file handle and the NAND's mount) with RAII
- Also split the file loading into another function that I will remove in a later PR

* Make NANDMount immovable

* Remove NAND-loading code that I had sectioned off into a function

- Incomplete copypasta
- I must have gotten distracted

* Tidy up NANDImage's initialization

- Don't unmount the disk image if the constructor fails (that's NANDMount's job now)
- Only assign CurFile if the constructor succeeds

* Add some const-correctness

* Move DSi NAND initialization to the frontend

- The NANDImage is now installed via a unique_ptr in DSi

* Remove Platform::DSi_NANDPath

- Not Config::DSiNANDPath; that can still be configured as usual
- The core no longer needs to care

src/FATStorage.cpp

@@ -21,6 +21,7 @@
 #include <inttypes.h>
 #include <vector>
 
+#include "FATIO.h"
 #include "FATStorage.h"
 #include "Platform.h"
 
@@ -122,7 +123,7 @@ UINT FATStorage::FF_ReadStorage(BYTE* buf, LBA_t sector, UINT num)
     return ReadSectorsInternal(FF_File, FF_FileSize, sector, num, buf);
 }
 
-UINT FATStorage::FF_WriteStorage(BYTE* buf, LBA_t sector, UINT num)
+UINT FATStorage::FF_WriteStorage(const BYTE* buf, LBA_t sector, UINT num)
 {
     return WriteSectorsInternal(FF_File, FF_FileSize, sector, num, buf);
 }
@@ -157,7 +158,7 @@ u32 FATStorage::ReadSectorsInternal(FileHandle* file, u64 filelen, u32 start, u3
     return res;
 }
 
-u32 FATStorage::WriteSectorsInternal(FileHandle* file, u64 filelen, u32 start, u32 num, u8* data)
+u32 FATStorage::WriteSectorsInternal(FileHandle* file, u64 filelen, u32 start, u32 num, const u8* data)
 {
     if (!file) return 0;