Struct representing a priority queue. The entries vector represents a max
heap structure, where entries[0] is the root, entries[1] and entries[2] are the
left child and right child of the root, etc. More generally, the children of
entries[i] are at at i * 2 + 1 and i * 2 + 2. The max heap should have the invariant
that the parent node’s priority is always higher than its child nodes’ priorities.