* 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
