Write data to SD in binary format

[wiktorwieclaw]

As suggested by @PatrykPaluch, we could store and send the data in binary format. Since Lora and SD are our biggest bottlenecks I think it's worthwhile to actually make the data as tiny as possible. There is a great crate for that called postcard that works with serde.

[wiktorwieclaw]

We may also think about compressing. I'll try to find a nice compression crate for no_std.

[wiktorwieclaw]

It turns out that compression without dynamic allocation is really hard. There is only one algorithm in crates.io called lzss that is suitable for bare-metal systems. It's a dictionary textual substitution algorithm though, so I don't think it would work well for us. It would probably make the logs longer.