v0.5.0

• Breaking: The Sortable trait now returns std::io::Result on both encode
  and decode methods, exposing underlying errors.
• Breaking: The SortedIterator iterator now returns std::io::Result<T>
  instead of T directly, allowing propagation of underlying errors.
• Breaking change: The comparator methods or key extractor now require being
  Clone. This change should not affect most users, as closures are Clone if
  they don't capture any variables.
• Added a new "pushed" iterator, which allows pushing new elements instead of
  consuming them through an iterator. This is particularly useful when the data is
  not readily available as an iterator.
• Methods accepting iterators now accept IntoIterator for flexibility.