Add support for complex data to PriorityQueue

[appleJax]

This PR adds support for complex data to the PriorityQueue class.

Changes:

• uses native JS Map data structure to store priorities. This way, objects, functions, or primitives can be used as keys. A plain JS Object coerces keys to strings, so different objects could all have the same key of [object Object]
• allows constructing a new PriorityQueue with a custom compareValue function, so complex data can be compared

example usage:

const JOB1 = { type: 'job1' };
const JOB2 = { type: 'job2' };
const JOB3 = { type: 'job3' };

const compareByType = (a, b) => {
  if (a.type === b.type) {
    return 0;
  }
  return a.type < b.type ? -1 : 1;
};

const priorityQueue = new PriorityQueue(compareByType);

priorityQueue.add(JOB2, 2);
priorityQueue.peek(); // returns JOB2 

priorityQueue.add(JOB3, 3);
priorityQueue.peek(); // returns JOB2 

priorityQueue.add(JOB1, 1);
priorityQueue.peek(); // returns JOB1

priorityQueue.changePriority(JOB2, 0);
priorityQueue.peek(); // returns JOB2 

const existingJobType = { type: 'job1' };
const newJobType = { type: 'job4' };

priorityQueue.hasValue(existingJobType); // returns true
priorityQueue.hasValue(newJobType); // returns false