/// Imports multiple items in parallel using thread::scope.
/// Each expression must return Result<T>.
///
/// # Example
/// ```ignore
/// let (a, b, c) = parallel_import! {
/// a = SomeVec::forced_import(&db, version),
/// b = OtherVec::forced_import(&db, version),
/// c = ThirdVec::forced_import(&db, version),
/// };
/// ```