PERF: resolution, is_normalized
```
dti = pd.date_range("2016-01-01", periods=10_000)
dta = dti._data

In [4]: %timeit dta.is_normalized
354 µs ± 3.62 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)   # <- main
122 µs ± 2.71 µs per loop (mean ± std. dev. of 7 runs, 10,000 loops each)  # <- PR

In [5]: %timeit dta.resolution
493 µs ± 4.86 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)  # <- main
258 µs ± 10.3 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)  # <- PR
```
Thanks @jbrockmendel 