Add AST dumper and improve upvalue analysis

The `UpvalueAnalyzer` now correctly identifies which lambdas capture
which variable declarations, rather than just marking declarations that
need boxing. This information is stored in a `capture_map`.

The `Binder` has been updated to use this new `capture_map` instead of a
`HashSet` of boxed declarations. The `DefLocal` node in `bound_nodes.rs`
now stores a `captured_by` field, which is a list of lambda identities
that capture the local variable.

A new `dumper` module has been added to provide a human-readable
representation of the bound AST, including information about captured
variables.

The `VM` has been updated to use the `captured_by` field to determine if
a local variable needs to be stored in a `Cell`, rather than relying on
a boolean `is_boxed` flag.

An example script `extreme_capture.myc` has been added to test deep
nesting and variable capture.

A new "Dump AST" button has been added to the UI, which uses the new
`Dumper` to display the bound AST.

src/ast/vm.rs

@@ -82,9 +82,9 @@ impl VM {
                 Ok(val)
             },
 
-            BoundKind::DefLocal { slot, value, is_boxed } => {
+            BoundKind::DefLocal { slot, value, captured_by } => {
                 let val = self.eval(value)?;
-                let final_val = if *is_boxed {
+                let final_val = if !captured_by.is_empty() {
                     Value::Cell(Rc::new(RefCell::new(val)))
                 } else {
                     val