feat: Use Cow<[T]> instead of Vec<T> where possible.

[duesee]

• https://git.meli.delivery/meli/meli/pulls/223#issuecomment-1161

[duesee]

This is harder than expected. Changing, e.g., Data::List { items: Vec<FlagNameAttribute<'a>> } to Data::List { items: Cow<'a, [FlagNameAttribute<'a>]> } results in the error message "the enum Cow<'a, B> is invariant over the parameter B".

I think what happens is that Cow::to_static(...) yields Vec<FlagNameAttribute<'a>> which isn't bounded by `'static'.

// $ cargo expand --features bounded-static

// ...

Data::List { items, delimiter, mailbox } => {
  Data::List {
      items: items.to_static(),
      delimiter: delimiter.to_static(),
      mailbox: mailbox.to_static(),
  }
}

// ...