PERF: improve perf of Series fastpath constructor
This came up while discussing the CoW proof of concept (and need for item_cache / speed of creating a Series of a column). Those small changes gives a decent reduction of some overhead (~30% reduction):

```python
arr = np.array([1, 2, 3])
idx = pd.Index(["a", "b", "c"])

pd.Series(arr, index=idx, name="name", fastpath=True)
```

```
In [5]: %timeit pd.Series(arr, index=idx, name="name", fastpath=True)
10.9 µs ± 202 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)  # <-- master
7.48 µs ± 187 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)  # <-- PR
```