message: Use hardline instead of line
  This avoids extra `FlatAlt` usages under the hood.
diff: @@ -281,10 +281,10 @@ impl<'interner, 'arena> Context<'interner, 'arena> {
  start_delim.clone(),
  self.concat(
  docs.clone()
  - .map(|doc| self.concat([self.line(), doc, separator.clone()])),
  + .map(|doc| self.concat([self.hardline(), doc, separator.clone()])),
  )
  .nest(INDENT),
  - self.line(),
  + self.hardline(),
  end_delim.clone(),
  ]),
  self.concat([
  