Return Result<Object,Object> from eval

Add Object::new_error function

Simplify imports in evaluator

Fix typo in lambda check

Use is_err instead of destructuring

Use little helper method in Object

Refactor `apply` in evaluator

Use type alias for Rc<RefCell<Environment>>

Simplify iterator in Environment::new()

Add builtin minus function to evaluator

Fix warnings

Add support for user-defined functions

Little cleanup after running Clippy

Clean up eval

Move `define` into evaluator and fix apply/eval