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.h

@@ -55,10 +55,10 @@ private:
     static Platform::FileHandle* FF_File;
     static u64 FF_FileSize;
     static UINT FF_ReadStorage(BYTE* buf, LBA_t sector, UINT num);
-    static UINT FF_WriteStorage(BYTE* buf, LBA_t sector, UINT num);
+    static UINT FF_WriteStorage(const BYTE* buf, LBA_t sector, UINT num);
 
     static u32 ReadSectorsInternal(Platform::FileHandle* file, u64 filelen, u32 start, u32 num, u8* data);
-    static u32 WriteSectorsInternal(Platform::FileHandle* file, u64 filelen, u32 start, u32 num, u8* data);
+    static u32 WriteSectorsInternal(Platform::FileHandle* file, u64 filelen, u32 start, u32 num, const u8* data);
 
     void LoadIndex();
     void SaveIndex();