Refactor AST nodes to use Rc

This commit refactors various AST node types to use `Rc` (Reference
Counting) instead of `Box`. This change is primarily driven by the need
for shared ownership and more efficient handling of potentially
recursive or shared data structures within the AST.

The main benefits of this change include:

- **Reduced cloning:** `Rc` allows multiple parts of the AST to refer to
  the same node without needing to clone the entire node. This is
  particularly beneficial for optimization passes where nodes might be
  visited multiple times.
- **Enabling sharing:** It makes it easier to represent scenarios where
  a single AST node might be a child of multiple parent nodes.
- **Performance improvements:** By avoiding unnecessary deep copies of
  AST nodes, this change can lead to performance gains, especially
  during complex compilation phases.

src/ast/vm.rs

@@ -893,7 +893,7 @@ impl VM {
     fn eval_and_flatten_internal<O: VMObserver>(
         &mut self,
         obs: &mut O,
-        elements: &[ExecNode],
+        elements: &[Rc<ExecNode>],
         into: &mut Vec<Value>,
     ) -> Result<(), String> {
         for e in elements {