Replace fatfs::io traits with embedded-io traits

[MabezDev]

This slotted in fairly easily, due to how similar the two sets of traits were. The only real difference is how UnexpectedEof is handled. Previously this crate had methods on the IoError trait to create an UnexpectedEof error, in embedded-io the read_exact method actually returns an entirely different enum, one which handles the UnexpectedEof or other io errors.

I believe this is a good change, for a number of reasons. Firstly embedded-io is almost a drop-in solution, and by switching more of the ecosystem can use this crate. embedded-io is established in the community and growing in popularity. embedded-io has a set of async traits which may allow this crate to take advantage of async io in the future.

[MabezDev]

Hmm, I guess one small downside to this is that we need edition 2021, I'll bump that and the MSRV too. Actually, I think its just a CI error, thought I can't seem to figure out what's causing it?

[MabezDev]

Converting to draft as embedded-io needs to properly support WriteZero errors, relevant PR: embassy-rs/embedded-io#18

[Dirbaio]

the io traits (both std::io and embedded-io) represent "any byte stream". Perhaps a "block device" trait would make more sense here?

For example, the contract of the io traits doesn't specify all seeks/reads/writes have to be aligned to the block size, but for io implementations for block devices they have to be...

[MabezDev]

the io traits (both std::io and embedded-io) represent "any byte stream". Perhaps a "block device" trait would make more sense here?

Good point! When I used version 0.3 of this crate with core_io, I ended up having to write a BufferredSdCard Wrapper which would ensure that any byte stream access went through as aligned block size reads and writes. I guess it is up to @rafalh whether it should be handled inside rust-fatfs as a block device or if the wrapper approach would be preferred.

[Dominaezzz]

Wrapper approach is probably better as I wouldn't want to have to conform to aligned reads for non block devices like EEPROMs.

[MabezDev]

FYI embedded-io has now been moved into rust-embedded/embedded-hal, where it will be used for the Serial traits and general I/O (rust-embedded/embedded-hal#466), further cementing the value of the crate.

[Dominaezzz]

0.5.0 was recently released so WriteZero is now available for use here

[MabezDev]

I won't be updating this PR any longer, as I have decided to hard fork to add async support here: https://github.com/MabezDev/embedded-fatfs. My fork uses the embedded-io traits, if you need blocking support I'd happily accept a PR which adds a blocking layer over the async API.

I hope this PR is still useful, anyone is welcome to salvage it to get e-io support in rust-fatfs.