text: Title: [ENH] Series toset() functionality
  Body: # Brief Description
  
  <!-- Please provide a brief description of what you'd like to propose. -->
  
  I would like to propose toset() functionality similar to [tolist()](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.tolist.html). 
  Basically it will be a function call to tolist and then conversion to set.
  Note: if the collection has no hashable member it will raise an exception.
  
  # Example API
  
  <!-- One of the selling points of pyjanitor is the API. Hence, we guard the API very carefully, and want to
  make sure that it is accessible and understandable to many people. Please provide a few examples of what the API
  of the new function you're proposing will look like. We have provided an example that you should modify. -->
  
  Please modify the example API below to illustrate your proposed API, and then delete this sentence.
  
  ```python
  # convert series to a set
  df['col1'].toset()
  
  