Consider adding `Eq` where possible

[brycx]

When #[warn(clippy::derive_partial_eq_without_eq)] is enabled, clippy will suggest to add Eq to some types. We should make sure this makes sense before we add this, because removing them later will be a breaking change.

[vlmutolo]

The additional semantic requirements for Eq are laid out in the trait's docs. I think this is okay to implement at least for any of our types that are just "bags of bytes". The three relevant properties are:

• reflexivity: a == a
• symmetry: a == b implies b == a
• transitivity: a == b and b == c implies a == c

The type system will enforce that the bag-of-bytes types can only be compared with the same type, which guarantees that the underlying bytes semantically mean the same thing and that we're not, for example, comparing a private key and a hash. That's the only thing I can think of that might break one of the above properties, and it's not an issue.

[brycx]

Just for reference, clippy suggests the additional Eq on the following types:

• X25519 PublicKey + PrivateKey + SessionKeys (high-level API)
• BLAKE2b Hasher enum

Good points @vlmutolo. Seems like this shouldn't be too much of an issue then.

[vlmutolo]

In the const generics refactor, the byte containers will all look like Public<C, D>. I think we can derive Eq on PublicData and the various D storage types (e.g. ArrayData). This means that Public will implement Eq if and only if the context parameter C implements Eq. So we can control which types implement Eq just using the context parameter, which is in line with the spirit of the refactor: pushing logic (such as min, max sizes) into the context parameters and letting everything else be the same.

I imagine we'll want to implement Eq for all the current byte container types, but it's good that we'll have the option to not implement it selectively.

[brycx]

Do you think it'd be better to postpone this to post const-generics, that you've been working on @vlmutolo?

[vlmutolo]

I think we can go ahead and add this in the current implementation. I'll also make sure to add it in the const-generics branch.