Improve byte array serialization

[robin-nitrokey]

Per default, byte arrays are serialized inefficiently by serde.

serde_bytes provides workarounds for byte slice serialization but does not support arrays yet. heapless_bytes improves the serialization for arrays but does not guarantee their length. Therefore we introduce a helper type, BytesArray, that uses heapless_bytes’ serialization but enforces a constant length.

serde-byte-array provides a wrapper type with a more efficient serialization format.

Fixes #11

[sosthene-nitrokey]

It's a bit of a pain that dtolnay completely ignores my pr to serde_bytes.

Maybe we should publish a fork of serde-bytes as serde-bytes-array? We already needed something like this in opcard (though we don't anymore).

[sosthene-nitrokey]

I would rather take the approach of wrapping a [u8;N] and manually implementing Serialize, Deserialize and Deref on them (which I think also gives us the ConstantTimeEq implementation. This avoids the unnecessary overhead of the Vec and the runtime checks.

Maybe just take my existing implementation?

[robin-nitrokey]

That was my first approach, but I’d rather not manually implement Deserialize here. That just duplicates code from serde-bytes or heapless-bytes and requires more testing than just wrapping Bytes.

[robin-nitrokey]

Maybe we should publish a fork of serde-bytes as serde-bytes-array? We already needed something like this in opcard (though we don't anymore).

Does this have to be a fork? Can’t we have a serde-byte(s)-array crate that just provides the Byte(s)Array type and, if needed, depends on serde-bytes?

[sosthene-nitrokey]

It doesn't necessarily have to be a fork no. We could have just the [u8;N] support in its own crate.
Maybe it could be part of heapless-bytes?

[robin-nitrokey]

I’d prefer a separate crate. Mabye other people are also interested in using this specialization without pulling in heapless.

[sosthene-nitrokey]

I pushed one at https://github.com/Nitrokey/serde-byte-array

[robin-nitrokey]

I pushed one at https://github.com/Nitrokey/serde-byte-array

404 – is it a private repository?

[sosthene-nitrokey]

I made it public
Should we publish it on crates.io?

[robin-nitrokey]

Thanks! I’ll make a PR with some minor changes and then we can release.