Release 0.2.0: Major API improvements

[AngelicosPhosphoros]

API

• Trait Clone is no more required for keys (Since it stored only once)
• Renamed method remove_item to remove
• Added method remove_entry which returns both key and priority
• push operation returns old priority now if same key already exists
• Method set_priority returns Result<TPriority, SetPriorityNotFoundError> with old priority instead of panicing on missing keys
• Added Entry API to allow whole cycle Find -> Read -> Update with just one hashmap lookup.
• Added borrowing unordered iterator (by method iter) over which will iterate over whole queue in O(n)
• Improved documentation by a little
• Added #[forbid(unsafe_code)]

  Implementation

• Now uses IndexMap from indexmap crate internally