Investigate other kinds of heaps for dijkstra_shortest_paths

[Douglas Gregor]

There are many types of heaps that provide good theoretical 
performance when used as a priority queue, e.g., by Dijkstra's 
shortest paths algorithm. The BGL contains implementations of two 
such data structures: a binary heap and a relaxed heap. The latter is 
used because it has been shown to provide the best overall 
performance. However, we should investigate other kinds of heaps, 
e.g., Fibonacci heaps, relaxed Fibonacci heaps, and run-relaxed 
heaps, to determine which option is best for the BGL.

Dietmar Kuhl implemented several kinds of heaps in his Heaps 
library, which could be found in earlier versions of Boost. It was 
removed from Boost in the 1.27.0 timeframe because it had never 
been reviewed and was not maintained, but may still prove valuable 
for experiments. 

[Douglas Gregor]

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"The lore" seems to indicate that relaxed heaps are good enough. If we find 
performance problems later or someone codes a better priority queue, we'll 
consider it at that time.