Improvement TODOs

[jchenche]

Things to do to make the codebase better:

• Merge two ASTs into one AST.
• Leverage the type-checker more (lot of pattern-matching cases are duplicated in type-checking and evaluation).
• Add unit and e2e tests.
• Enable multi-dimensional arrays.
• Add lines for errors.
• Add example code for documentation.
• Add typing rules and formal semantics.