message: Replace plan::validate::Context.labels item type from String to str
diff: @@ -43,7 +43,7 @@ pub fn validate(schema_map: &SchemaMap, statement: &Statement) -> Result<()> {
  enum Context<'a> {
  Data {
  - labels: Option<Vec<&'a String>>,
  + labels: Option<Vec<&'a str>>,
  next: Option<Rc<Context<'a>>>,
  },
  Bridge {
  @@ -53,7 +53,7 @@ enum Context<'a> {
  }
  impl<'a> Context<'a> {
  - fn new(labels: Option<Vec<&'a String>>, next: Option<Rc<Context<'a>>>) -> Self {
  + fn new(labels: Option<Vec<&'a str>>, next: Option<Rc<Context<'a>>>) -> Self {
  Self::Data { labels, next }
  }
  @@ -100,11 +100,11 @@ impl<'a> Context<'a> {
  }
  }
  -fn get_lables(schema: &Schema) -> Option<Vec<&String>> {
  +fn get_lables(schema: &Schema) -> Option<Vec<&str>> {
  schema.column_defs.as_ref().map(|column_defs| {
  column_defs
  .iter()
  - .map(|column_def| &column_def.name)
  + .map(|column_def| column_def.name.as_str())
  .collect::<Vec<_>>()
  })
  }
  