Categorical optimization

As the `odo` issue https://github.com/blaze/odo/issues/561 mentions, a bottleneck emerged with respect to `datashape.Categorical` instantiation pointing to [this line](https://github.com/blaze/datashape/blob/master/datashape/coretypes.py#L1106), where the constructor coerces the input categories into a tuple. I wonder whether we should relax this constraint for Categorical objects, such that we could represent the underlying categories as numpy arrays, i.e. `Series.cat.categories.values`, and speed up `datashape` with respect to pandas/dask categorical discovery. cc @jbednar @teoliphant