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/DSi_AES.h

@@ -26,12 +26,12 @@
 #pragma GCC diagnostic ignored "-Wattributes"
 #if defined(__GNUC__) && (__GNUC__ >= 11) // gcc 11.*
 // NOTE: Yes, the compiler does *not* recognize this code pattern, so it is indeed an optimization.
-__attribute((always_inline)) static void Bswap128(void* Dst, void* Src)
+__attribute((always_inline)) static void Bswap128(void* Dst, const void* Src)
 {
     *(__int128*)Dst = __builtin_bswap128(*(__int128*)Src);
 }
 #else
-__attribute((always_inline)) static void Bswap128(void* Dst, void* Src)
+__attribute((always_inline)) static void Bswap128(void* Dst, const void* Src)
 {
     for (int i = 0; i < 16; ++i) 
     {