REF: mix NDArrayBacked into Categorical, PandasArray

[jbrockmendel]

• closes #xxxx
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

[jreback]

does this have any perf implications?

pls rebase

[jbrockmendel]

does this have any perf implications?

In [3]: cat = pd.Categorical(range(5))

In [4]: %timeit cat.copy()
315 ns ± 5.19 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)  <-- PR
802 ns ± 7.08 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)  <-- master

[jbrockmendel]

Rebased + green

[jreback]

great!