ENH: Add dtype-support for pandas' type-hinting
It would be great if you could annotate a series, for example with - 

```python
def process_series(ser: pd.Series[int, str]) -> pd.Series[int, int]:
```

to indicate that the function should accept a string-valued series with an integer index, and output
int-valued series with an integer index. 
It could be even better if you would build on that such that the series member's types could be inferred using `TypeVar`s (for example, `pd.Series[int,str].iloc` would accept an integer and return a string), but that's not necessary - just a bonus or a later milestone. 

To do that, you could add a new module (maybe "pandas.typing"?) that would contain these type-hints and would require minimal integration (if any) into the pandas' infrastructure. There's a similar package for "numpy" that's external to it called [nptyping](https://github.com/ramonhagenaars/nptyping) that could be used as a reference.