Change to incremental GC

src/stack.h

@@ -24,6 +24,8 @@ struct StackFrame {
     LispVal *args; // arguments of the function call
     LispVal *lexenv; // lexical environment (plist)
     struct LocalReferences local_refs;
+
+    bool marked; // whether we have GC'ed this frame
 };
 
 struct LispStack {
@@ -69,6 +71,8 @@ static inline void new_lexical_variable(LispVal *name, LispVal *value) {
     assert(the_stack.depth != 0);
     LISP_STACK_TOP()->lexenv =
         CONS(name, CONS(value, LISP_STACK_TOP()->lexenv));
+    LISP_STACK_TOP()->marked = false;
+    gc_mark_stack_for_rescan();
 }
 
 // Copy the previous frame's lexenv to the top of the stack.