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/environment.rs

@@ -8,6 +8,8 @@ use crate::ast::vm::VM;
 
 use crate::ast::compiler::tco::TCO;
 
+use crate::ast::compiler::dumper::Dumper;
+
 pub struct Environment {
     pub global_names: Rc<RefCell<HashMap<String, (u32, StaticType)>>>,
     pub global_values: Rc<RefCell<Vec<Value>>>,
@@ -105,6 +107,16 @@ impl Environment {
         });
     }
 
+    pub fn dump_ast(&self, source: &str) -> Result<String, String> {
+        let mut parser = Parser::new(source)?;
+        let untyped_ast = parser.parse_expression()?;
+        
+        let bound_ast = Binder::bind_root(self.global_names.clone(), &untyped_ast)?;
+        let optimized_ast = TCO::optimize(bound_ast);
+        
+        Ok(Dumper::dump(&optimized_ast))
+    }
+
     pub fn run_script(&self, source: &str) -> Result<Value, String> {
         // 1. Parse
         let mut parser = Parser::new(source)?;