ExtensionArray.map
Both Categorical and SparseArray found implementing a `.map` method useful. This allows them to efficiently apply a function / mapping to the categories / sp_values, rather than every element of an array. We dispatch to it internally in https://github.com/pandas-dev/pandas/blob/master/pandas/core/series.py#L3379-L3380

So, we need to either

1. Add it to the interface
2. hard-code checks for categorical or sparse dtype there.

Do people have a preference? Right now I'm leaning toward 2. Or are there other array types that would have a similar efficiency gain to Categorical or Sparse?