A* heuristic based on distance to the goal

[ragoune]

Compute the distance to the goal using the current state value, instead of a simple incrementation using the number of steps from the starting point.
In the benchmark it reduces the required loop number from 46 to 14. Benchmarks:
Before:

goos: linux
goarch: amd64
pkg: benchmark
cpu: AMD Ryzen 7 5800X 8-Core Processor             
BenchmarkGoapAI-16                 49186             25324 ns/op           12777 B/op        449 allocs/op

After:

goos: linux
goarch: amd64
pkg: benchmark
cpu: AMD Ryzen 7 5800X 8-Core Processor             
BenchmarkGoapAI-16                129136              9122 ns/op            8288 B/op        166 allocs/op

Additional improvements:

• Heap storage for the nodes, to reduce the number of slices & maps. Reduces the fetch of next node
• One call to fetchNodeInHeap instead of two
• Hash with XOR incrementation
• Simplified hash using a simple hash multiplication
• Unit Test coverage > 90%
• Code documentation