Add Async::PriorityQueue for Consumer-Priority Resource Allocation.

[samuel-williams-shopify]

This PR introduces Async::PriorityQueue, a unique concurrency primitive that implements the inverse of traditional task prioritization. Instead of prioritizing work items, it prioritizes the consumers requesting work, enabling sophisticated resource allocation strategies based on who is asking rather than what is being processed.

• Worker Class Hierarchies: Critical system processes bypass background job consumers.
• Load Balancing: High-priority services get first access to shared resource pools.
• Capacity Management: Important consumers reserve access to constrained resources.

Example

Sync do
	queue = Async::PriorityQueue.new
	
	# Start consumers with different priorities:
	Async {puts queue.dequeue(priority: 1)}   # Low priority
	Async {puts queue.dequeue(priority: 10)}  # High priority
	
	# High priority consumer gets this first:
	queue.push("item")
end

Types of Changes

• New feature.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[franciscojmc]

Consideration: Handling Dead Fibers in the Waiter Queue

Great implementation. I wanted to raise a potential edge case for discussion regarding fiber lifecycle management.

Scenario to consider:

What happens if a fiber dies unexpectedly while waiting in the queue? Currently, the dead fiber's Waiter would remain in the priority heap, which could lead to items being lost:

queue = Async::PriorityQueue.new

# Fiber starts waiting
fiber = Async do
  queue.dequeue(priority: 10)
end

# Fiber terminates unexpectedly
fiber.stop

# The Waiter object remains in @waiting heap

# Later, when we push an item:
queue.push("item1")
# This would:
# 1. Pop the dead fiber's waiter from heap
# 2. Assign "item1" to the dead waiter
# 3. Signal a condition variable no one is waiting on
# Result: "item1" is effectively lost

What are your thoughts on whether this should be handled?

[ioquatix]

This implementation is both fiber and thread safe.

[ioquatix]

LGTM.