Assess speed up due to heap structure

[razziel89]

The current implementation that simply iterates over a graph is not as efficient as using a binary heap. Assess speed up by using a heap structure.

Greatly improve speed up by making nodes aware of which graph they are in. That way, a node can only be in one graph, but that's OK as it greatly improves performance. We will get rid of the map entirely.

[razziel89]

Turns out a heap structure is a lot more efficient than the current graph implementation, at least when it comes to addition and removal of the cheapest element. But a heap, as currently implemented via a simple slice, is not really efficient when it comes to determining whether an element exists in it.

In #12, a heap will be implemented that is a hybrid of a struct and a map. The struct will be used for efficient addition and removal of elements in sorted order while the map will be used to determine whether an element exists.