Change builtin functions mangling and their return type flattening

• the new mangling is more consistent with the language itself
• this allows more user-friendly error messages

src/language/ast/ASTNodeAffectationExpressionBuilder.cpp

@@ -43,6 +43,6 @@ ASTNodeAffectationExpressionBuilder::ASTNodeAffectationExpressionBuilder(ASTNode
     error_message << "undefined affectation type: ";
     error_message << rang::fgB::red << affectation_name << rang::fg::reset;
 
-    throw ParseError(error_message.str(), std::vector{node.begin()});
+    throw ParseError(error_message.str(), std::vector{node.children[0]->begin()});
   }
 }