Add local variable collection to compilation

* Upon entering a new body, the local scope variables are collected.
* VM instruction PushLocal now uses a `Local` value instead of a `Sym`
  value
* Compilation of a new AST body will implicitly create a new scope

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/compile/thunk.rs

@@ -213,7 +213,10 @@ impl<'c, 't> CompileBody<'c, 't> {
     }
 
     pub fn compile(&mut self, body: &'c Body) -> Result<Thunk> {
-        self.visit_body(body)
+        self.compile.push_scope_layer();
+        let thunk = self.visit_body(body)?;
+        self.compile.pop_scope_layer();
+        Ok(thunk)
     }
 }
 
@@ -228,6 +231,7 @@ impl Visit for CompileBody<'_, '_> {
     type Out = Result<Thunk>;
 
     fn visit_body(&mut self, body: &Body) -> Self::Out {
+        self.compile.collect_locals(body);
         let mut thunk = Thunk::Nop;
 
         for stmt in body.body.iter() {
@@ -342,14 +346,10 @@ impl Visit for CompileBody<'_, '_> {
     fn visit_atom(&mut self, atom: &Atom) -> Self::Out {
         let thunk = match atom {
             Atom::Ident(ident) => {
-                // TODO : set up lexical name stack
-                //  - think about how lexical names in function defs need to be captured, so no
-                //   references get prematurely GC'd
-                //  - Python uses LOAD_CLOSURE and CREATE_FUNCTION ops, but is this necessary if we
-                //    know the names being closed over at runtime? Hm.
-
+                let sym = global_sym(ident.to_string());
+                let local = self.compile.lookup_scope(sym).unwrap();
                 // get local
-                Inst::PushLocal(global_sym(ident.to_string())).into()
+                Inst::PushLocal(local).into()
             }
             Atom::Sym(sym) => {
                 // push symbol