message: style: reword ast::print docs
  Also, small formatting fix and removal of debugging test.
diff: // except according to those terms.
  /*!
  -This module provides a regular expression printer.
  +This module provides a regular expression printer for `Ast`.
  */
  use std::fmt;
  @@ -434,11 +434,6 @@ mod tests {
  assert_eq!(given, dst);
  }
  - #[test]
  - fn scratch() {
  - roundtrip(".");
  - }
  -
  #[test]
  fn print_literal() {
  roundtrip("a");
  