FAT32 adapters

[ost-ing]

I've noticed that the FAT32 adapter for the sdmmc-embedded crate was added to the sdmmc.rs source file.

I would suggest we move this into its own dedicated file, such as fat32.rs, as perhaps apart of an some kind of extensions folder. These adapters aren't strictly driver implementations and so should probably be separated.

I would then add an adapter for the alternative rust-fatfs crate.

Wdyt?

[olback]

I think this would be a good change. It'd also be nice to implement FAT support for EMMC.