sagemathgh-39228: faster bidirectional dijkstra using pairing heap
    
We improve methods `bidirectional_dijkstra` and
`bidirectional_dijkstra_special` from `c_graph.py` by using the new
pairing heap data structure (see sagemath#39046) instead of a `priority_queue`.


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### ⌛ Dependencies

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URL: sagemath#39228
Reported by: David Coudert
Reviewer(s): David Coudert, gmou3

Author: Release Manager

src/sage/data_structures/pairing_heap.pxd

@@ -30,6 +30,7 @@ cdef extern from "./pairing_heap.h" namespace "pairing_heap":
         void decrease(TypeOfItem, TypeOfValue) except +
         bint contains(TypeOfItem)
         TypeOfValue value(TypeOfItem) except +
+        size_t size()
 
     cdef cppclass PairingHeapNodePy:
         PyObject * value           # value associated with the item