Add tiebreak support to A* implementation

[cwoodhayes]

Currently the A* implementation in this crate doesn't support a tie-breaking function, which would allow customizing the behavior when several nodes at the front of the queue share the same priority value.

According to wikipedia, tiebreak selection can significantly impact performance--I ran across this myself in a side project.

I'd be happy to open a PR for this change!

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Also thanks so much for this great library :)

[samueltardieu]

Hi @cwoodhayes. Do you have an idea for how to do that without modifying the public API of astar/idastar?