concatenation wrappers

[ExpandingMan]

I would like the ability to concatenate ArrowVectors. Indeed, this is probably required to support many formats. I was thinking something like

struct Concatenation{J,P<:ArrowVector} <: ArrowVector
    comps::Vector{P}
end

Another alternative would be to create an ArrowVector with multiple index pointers, but that seems like it would get far more complicated, and the above takes better advantage of existing code.