PERF: RangeIndex.insert
```
index = pd.Index(range(2))

%timeit index.insert(2, 2)
48.4 µs ± 2.03 µs per loop (mean ± std. dev. of 7 runs, 10000 loops each)  # <- master
2.54 µs ± 70.1 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)  # <- PR
```