feat: lookup at downgrade time

works, but leaks memory

src/bytecode.rs

@@ -16,6 +16,8 @@ pub enum OpCode {
     Const { idx: usize },
     /// load a dynamic var onto stack
     LookUp { sym: usize },
+    /// load a var from let binding onto stack
+    LookUpLet { level: usize, idx: usize },
     /// load a thunk lazily onto stack
     LoadThunk { idx: usize },
     /// load a thunk value onto stack
@@ -31,6 +33,8 @@ pub enum OpCode {
     Call,
     /// make a function
     Func { idx: usize },
+    /// load a function argument
+    Arg { level: usize },
 
     /// consume 1 element, assert TOS is true
     Assert,
@@ -43,14 +47,14 @@ pub enum OpCode {
     /// push an empty attribute set onto stack
     AttrSet { cap: usize },
     /// finalize the recursive attribute set at TOS
-    FinalizeRec,
+    FinalizeLet,
     /// [ .. set value ] consume 1 element, push a static kv pair (`name`, `value`) into `set`
     PushStaticAttr { name: usize },
     /// [ .. set name value ] consume 2 elements, push a dynamic kv pair (`name`, `value`) in to `set`
     PushDynamicAttr,
 
     /// push an empty list onto stack
-    List,
+    List { cap: usize },
     /// [ .. list elem ] consume 1 element, push `elem` into `list`
     PushElem,
 
@@ -75,8 +79,6 @@ pub enum OpCode {
     SelectDynamic,
     /// [ .. set sym default ] select `sym` from `set` or `default`
     SelectDynamicOrDefault,
-    /// enter the let environment of the attribute set at TOS
-    EnterLetEnv,
     /// enter the with environment of the attribute set at TOS
     EnterWithEnv,
     /// exit current envrironment