Allow categoricals/enums backed by smaller int dtypes

[mcrumiller]

Description

Matlab's categorical arrays are always backed by the smallest integer type that can support the number of categories. While this isn't necessarily the best implementation (adding a new category after the limit is reached requires an upcast), it helps a lot with performance when the number of categories is small--for example, using a u8 to represent your categories can help with space.

I was wondering what people thought about either:

1. Taking the Matlab approach, where the user never touches the underlying type, but that type is dynamic; or
2. Allowing the underlying int type to be specified during construction, i.e. pl.Categorical(dtype=pl.UInt8).