ENH: add support dtype='category' in Series constructor

xref #8074
closes #8076

```
In [1]: Series([1,1,2,2,3,4,5],dtype='category')
Out[1]: 
0    1
1    1
2    2
3    2
4    3
5    4
6    5
dtype: category
Levels (5, int64): [1 < 2 < 3 < 4 < 5]
```
Oups, not LGTM: if we have a fill value, it should error if that value is not in levels and if it is the fill value should be translated into that code.