conversion wrappers

[ExpandingMan]

I'd like to add some "conversion wrappers" i.e. something like

struct Converter{J,P<:ArrowVector} <: ArrowVector{J}
    base::P
end

which automatically converts the data in base to be of type J. This seems a little elaborate, but I still think I prefer this to adding the functionality to the base ArrowVectors because I think there is a virtue in keeping their deserialization simple.

This would be particularly useful for Arrow.Datestamp and Arrow.Timestamp.

I'm of course open to other suggestions.