Split up internal_types.cpp into separate files

Use map<> to handle the ENUM <-> STRING mappings instead of IF/ELSE statements

Split up Transformer into separate files

Wrap Transformer functions in Transformer class rather than being top-level functions

Split up ExpressionExecutor into separate files (one file per expression type)

Organize logical_operator and physical_operator and expression into separate directories (e.g. joins, scans, etc)

expression_rules/logical_rules -> rules/expression/, rules/logical/

Consistent plural/singular -> directories are singular (“function” not “functions”, etc)

Prefix all test files with "test_"

Exclude test files from code sanity analysis (?)