Fix up compile in compiler blog part 6, and add more text.

06/env.hpp

@@ -11,7 +11,7 @@ struct env {
 
 using env_ptr = std::shared_ptr<env>;
 
-struct env_var {
+struct env_var : public env {
     std::string name;
     env_ptr parent;
 
@@ -22,7 +22,7 @@ struct env_var {
     bool has_variable(const std::string& name) const;
 };
 
-struct env_offset {
+struct env_offset : public env {
     int offset;
     env_ptr parent;