Refactor bound node types for clarity

Introduce generic `CompilerPhase` trait to unify different stages of the
bound AST. Rename `LocalSlot` to `VirtualId` and introduce `StackOffset`
for clearer distinction between compile-time and run-time addressing.
Update type aliases and implementations to reflect these changes.

src/ast/vm.rs

@@ -1,5 +1,4 @@
-use crate::ast::compiler::bound_nodes::{Address, AnalyzedNode, BoundKind};
-use crate::ast::compiler::lowering::ExecNode;
+use crate::ast::compiler::bound_nodes::{Address, AnalyzedNode, BoundKind, ExecNode, StackOffset};
 use crate::ast::types::{Object, Value};
 use std::any::Any;
 use std::cell::RefCell;
@@ -335,7 +334,7 @@ impl VM {
                     val.clone()
                 };
 
-                self.set_value(addr.clone(), store_val)?;
+                self.set_value(*addr, store_val)?;
                 // Define always evaluates to the unwrapped value for immediate use.
                 Ok(val)
             }
@@ -351,7 +350,7 @@ impl VM {
                 }
                 Ok(val)
             }
-            BoundKind::Get { addr, .. } => self.get_value(addr.clone()),
+            BoundKind::Get { addr, .. } => self.get_value(*addr),
 
             BoundKind::FieldAccessor(k) => Ok(Value::FieldAccessor(k.clone())),
 
@@ -399,7 +398,7 @@ impl VM {
 
             BoundKind::Set { addr, value } => {
                 let val = self.eval_internal(obs, value)?;
-                self.set_value(addr.clone(), val.clone())?;
+                self.set_value(*addr, val.clone())?;
                 Ok(val)
             }
             BoundKind::If {
@@ -882,7 +881,7 @@ impl VM {
         }
     }
 
-    fn capture_upvalue(&mut self, addr: Address) -> Result<Rc<RefCell<Value>>, String> {
+    fn capture_upvalue(&mut self, addr: Address<StackOffset>) -> Result<Rc<RefCell<Value>>, String> {
         match addr {
             Address::Local(slot) => {
                 let frame = self.frames.last().ok_or("No call frame")?;
@@ -918,7 +917,7 @@ impl VM {
         }
     }
 
-    fn get_value(&self, addr: Address) -> Result<Value, String> {
+    fn get_value(&self, addr: Address<StackOffset>) -> Result<Value, String> {
         match addr {
             Address::Local(slot) => {
                 let frame = self.frames.last().ok_or("No call frame")?;
@@ -958,7 +957,7 @@ impl VM {
         }
     }
 
-    fn set_value(&mut self, addr: Address, value: Value) -> Result<(), String> {
+    fn set_value(&mut self, addr: Address<StackOffset>, value: Value) -> Result<(), String> {
         match addr {
             Address::Local(slot) => {
                 let frame = self.frames.last().ok_or("No call frame")?;