implement reference offset

A number of binary formats contain dictionary encoded data with the references as 0-indexed integers. PooledArrays currently can't be used to wrap these because, if I understand correctly, the references are always 1-based. This means that to deserialize a format using PooledArrays you necessarily have to copy all the references.

I suggest we add an offset::Int field so this can be handled more generally. This would be added getindex. The most obvious difficulty with implementing this is that currently zeros give undefined values. Perhaps this can be circumvented at compile time with a new type parameter.

Anyway, before I try to implement this, has anyone given in consideration? How is arrow (which I seem to remember is 0-based) deal with this?

Start by examining PooledArrays' getindex behavior and how references are represented, especially the current treatment of zero values. Investigate how Arrow handles 0-based references, then determine whether an offset can support them without copying while preserving undefined-value behavior; no file or test is named in the issue.