message: core: add a Display implementation for Level
  ## Motivation
  Provide a way to get a clean display of a `Level`. It uses the same
  strings as displayed in fmt.
  Closes:
diff: @@ -261,6 +261,18 @@ impl Level {
  pub const TRACE: Level = Level(LevelInner::Trace);
  }
  +impl fmt::Display for Level {
  + fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  + match *self {
  + Level::TRACE => f.pad("TRACE"),
  + Level::DEBUG => f.pad("DEBUG"),
  + Level::INFO => f.pad("INFO"),
  + Level::WARN => f.pad("WARN"),
  + Level::ERROR => f.pad("ERROR"),
  + }
  + }
  +}
  +
  #[repr(usize)]
  #[derive(Copy, Clone, Debug, Eq, PartialEq, Hash, Ord, PartialOrd)]
  enum LevelInner {
  