BUG: isin numeric vs string
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Using the new hashtables!

```
In [3]: arr = np.arange(10**7).astype(np.int32)

In [4]: %timeit isin(arr, arr[:10])
88.8 ms ± 497 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)   # <-- master
42.7 ms ± 385 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)   # <-- PR
```