Introduce new `maxdepth` keyword argument for setting a limit on nesting (#147)

Introduce new `maxdepth` keyword argument for setting a limit on nesting (#147)

level limit

Alternative fix for #143. This is a more general fix than just
specializing CategoricalArrays. This should prevent more general cases
of the same issue: i.e. someone accidently passes a recursive data
structure and `Arrow.write` gets stuck trying to recursively serialize.