Adds PriorityQueue, ConcurrentPriorityQueue

Now you can construct max- and min- queues, both classical and thread-safe!

Bugfixes

Fixes bug with ConcurrentDictionary subscript nil assignment

Now uses read-write lock, added several methods

Methods added:
ConcurrentDictionary:
mutateValue(forKey key: Key, mutation: (Value) -> Value)

ConcurrentArray:
mutateValue(at index: Int, mutation: (Element) -> Element)
subscript(safe index: Int) -> Element?

Initial release

v1.1.0

Bump version

Initial release

v1.0.0

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