Add chunked read/write APIs

[sosthene-nitrokey]

These APIs will be useful for a trussed streaming API

[nickray]

Do you envision some kind of read_to_end method that allows "just" reading a file that's bigger than our interchange buffer? This would call read_chunk multiple times, possibly failing early. I think that would be very ergonomic for an app, however it might be hacky with the multiple syscalls involved.

[sosthene-nitrokey]

This doesn't make sense in littlefs2 I think, as in that case you can already use write_all and read_to_end.

I do add such a util in the trussed itself: https://github.com/Nitrokey/trussed/pull/18/files#diff-23af5356f6001cd3993cd3c801fb7716ea02d1e504081b9fb569332db6107e80R12