ENH: Make Series.explode work for sets

[dsaxton]

Currently Series.explode does nothing for sets but I think this would be just as useful as lists or tuples. It also allows for a way to easily enforce uniqueness at the row level before exploding.

I don't think this would be breaking, unless something is specifically relying on the operation not working.

import pandas as pd                                                                                                                                                                                  

ser = pd.Series([{1, 2}, {1, 2, 3}]) 
ser
# 0       {1, 2}
# 1    {1, 2, 3}
# dtype: object                                                                                                                                                               
ser.explode()                                                                                                                                                                                        
# 0       {1, 2}
# 1    {1, 2, 3}
# dtype: object

Desired output:

ser.explode()
# 0    1
# 0    2
# 1    1
# 1    2
# 1    3
# dtype: object