Make operators a singleton object instead of a class

[vxgmichel]

This has the benefit of not exposing the operator methods (raw and pipe) to the produced stream.

It was also confusing for operators to be a class as they're written in lowercase.

Also, the implementation of the operator and pipe_operator decorators are updated to be more readable.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (4952900) to head (102b780).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #106   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines         1058      1088   +30     
=========================================
+ Hits          1058      1088   +30     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.