Add basic lexenv support

src/stack.h

@@ -40,6 +40,15 @@ static ALWAYS_INLINE struct StackFrame *LISP_STACK_TOP(void) {
     return the_stack.depth ? &the_stack.frames[the_stack.depth - 1] : NULL;
 }
 
+static ALWAYS_INLINE LispVal *TOP_LEXENV(void) {
+    return the_stack.depth ? LISP_STACK_TOP()->lexenv : Qnil;
+}
+
+static ALWAYS_INLINE LispVal *PARENT_LEXENV(void) {
+    return the_stack.depth > 1 ? the_stack.frames[the_stack.depth - 2].lexenv
+                               : Qnil;
+}
+
 void lisp_init_stack(void);
 void lisp_teardown_stack(void);
 void push_stack_frame(LispVal *name, LispVal *fobj, LispVal *args);
@@ -47,6 +56,14 @@ void pop_stack_frame(void);
 void add_local_reference_no_recurse(LispVal *obj);
 void add_local_reference(LispVal *obj);
 
+// Return true if successful, false if not found and not created
+bool set_lexical_variable(LispVal *name, LispVal *value, bool create_if_absent);
+// Just add a new lexical variable without any checking
+void new_lexical_variable(LispVal *name, LispVal *value);
+
+// Copy the previous frame's lexenv to the top of the stack.
+void copy_parent_lexenv(void);
+
 // used by the GC
 void compact_stack_frame(struct StackFrame *restrict frame);