fixed some memory leaks

src/ast/expressions/if_then_else.cpp

@@ -20,4 +20,10 @@ namespace ast {
             ss << "else" << alternative->str();
         return ss.str();
     };
+
+    if_then_else::~if_then_else() {
+        delete condition;
+        delete consequence;
+        delete alternative;
+    }
 } // namespace ast

src/ast/expressions/if_then_else.hpp

@@ -15,5 +15,6 @@ namespace ast {
 
         std::string token_literal() const override;
         std::string str() const override;
+        ~if_then_else();
     };
 } // namespace ast