refactor operator checker

[topherinternational]

A bunch of refactor items:

• Break the OperatorChecker into more methods/functions
• Each pylint rule we implement has a corresponding check_<rule>() method, isolating the logic of the check from how we procure the nodes needed to compute the check result
• Test the check methods individually
• Use a custom dataclass for extracting the task/operator parameters from an assignment node
• Move the pylint MSGS to the top of each module for easier reading