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.

examples/extreme_capture.myc

@@ -0,0 +1,25 @@
+;; Output: 36
+(do
+  ;; Excessive capture test
+  ;; This script creates deep nesting where the inner-most lambda
+  ;; grabs variables from every single outer scope.
+
+  (def excessive (fn [v1]
+    (do
+      (def v2 2)
+      (fn [v3]
+        (do
+          (def v4 4)
+          (fn [v5]
+            (do
+              (def v6 6)
+              (fn [v7]
+                (do
+                  (def v8 8)
+                  ;; v1, v3, v5, v7 are parameters (captured as upvalues)
+                  ;; v2, v4, v6, v8 are locals (captured from outer scopes)
+                  (+ v1 v2 v3 v4 v5 v6 v7 v8))))))))))
+
+  ;; Execution: (1 + 2 + 3 + 4 + 5 + 6 + 7 + 8) = 36
+  ((((excessive 1) 3) 5) 7)
+)