Incorrect `iszero` behavior

```julia-repl
julia> using RecursiveArrayTools

julia> x = ArrayPartition(randn(2), rand(4))
([1.375871544469621, -1.3845891595359359], [0.5327588820305011, 0.12551033581941717, 0.6268906305772934, 0.4529256190041555])

julia> iszero(x)
true
```

This might have to do with incorrect `all` behavior, since `all(iszero, ...)` also gives incorrect result.