Handle incorrect VZV bytes representation

[robertbastian]

The empty VZV is currently either [], when created with VZV::new/VZS::new_empty, or [0,0,0,0] when created with VZV::from(&[]). This breaks the byte equality invariant that PartialEq, among others, relies on.

This fix makes VZS::new_empty return [0,0,0,0]. However, as there is data with the [] out in the wild, we probably have to adapt at least PartialEq to handle it.

This fix make VZV::from return [] for empty input, and updates the PartialEq implementation to deal with the non-canonical representation.

[Manishearth]

IMO empty VZVs should canonically be the empty slice

[Manishearth]

I'm fine allowing this to slip

[Manishearth]

though this is ok too, note that this also breaks the VarULE "equality is byte equality" invariant on VZS

[Manishearth]

if we remove this we should also special case is_empty to check for empty length

[robertbastian]

is_empty should already work, because it checks VZComponent's indices slice, which gets special-case initialised.

note that this also breaks the VarULE "equality is byte equality" invariant on VZS

We already broke that invariant

[robertbastian]

Note that testdata CI is failing so the incorrect representation is out there

[Manishearth]

Alright, that's fine by me then, since it's legacy-only. We may wish to deprecate this in the long run.

[sffc]

I slightly lean toward [0,0,0,0] being canonical because it's nice that a VZV is guaranteed to have these 4 length bytes (no special cases) but not a strong opinion. It's nice that [] saves us some space.