+ Keep track of the current index the Heap Element is
  located at in the heap so we find the Element in
  heap at a later time.

-- daf
+ Use the max size_t value as a sentinel, in order to determine
  if an Element is no longer in the heap.

-- daf
+ Implement Heap's IncreaseElementKey(...) method.

+ Provide associated unit test.

-- daf