PERF: CategoricalIndex.get_indexer

[jbrockmendel]

• closes PERF: regression in CategoricalIndex.get_indexer #42249
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

import pandas as pd
import string, itertools
data_unique = pd.CategoricalIndex(
            ["".join(perm) for perm in itertools.permutations(string.printable, 3)]
)
cat_list = ["a", "c"]

%timeit data_unique.get_indexer(cat_list)
106 µs ± 6.14 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)  # <- PR
39 ms ± 616 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)  # <- 1.2.4
290 ms ± 3.9 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)  # <- master

[jreback]

@meeseeksdev backport 1.3.x

[jreback]

cc @jbrockmendel guess this won't backport easily