pathfinding: relocated coordinate hashing to std namespace
datastructure/pairing_heap: created type member for heap nodes
pathfinding: utilize std::hash<coord::phys3> in nodemap
pathfinding: remove hardcoding for navigation grid size

now using 1/8 of the phys-resolution per game tile
pathfinding: moved cost comparison from a_star to path

this also fixes using the correct operator for the pairing heap:
< instead of >
pathfinding: use pairing_heap instead of std::priority_queue
pathfinding: code beautifications and comments