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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

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

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