sagemathgh-39300: faster method __mul__ for graphs

    
Instead of $n-1$ additions of graphs, we use a logarithmic number of
additions of graphs to compute $G * n$.

Before
```sage
sage: def test(G):
....:     for i in [1, 2, 3, 4, 5, 10, 15, 20, 50, 100]:
....:         t = walltime()
....:         for _ in range(10):
....:             H = G * i
....:         t = walltime() - t
....:         print(f"{i}\t {round(t/10, 5)}")
sage: test(Graph([(0, 1)]))
1        3e-05
2        0.00011
3        0.00016
4        0.00024
5        0.00032
10       0.00072
15       0.00108
20       0.00139
50       0.00437
100      0.01272
sage: test(graphs.PetersenGraph())
1        5e-05
2        0.00017
3        0.00028
4        0.00042
5        0.00055
10       0.00148
15       0.0024
20       0.00343
50       0.01802
100      0.07057
```

Now
 ```sage
sage: test(Graph([(0, 1)]))
1        3e-05
2        0.0001
3        0.00018
4        0.00018
5        0.00025
10       0.00035
15       0.00053
20       0.00043
50       0.00083
100      0.00118
sage: test(graphs.PetersenGraph())
1        4e-05
2        0.00015
3        0.00028
4        0.00026
5        0.00041
10       0.00066
15       0.00117
20       0.00094
50       0.0024
100      0.00448
```

### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: sagemath#39300
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton

src/sage/graphs/generic_graph.py

@@ -763,20 +763,40 @@ def __mul__(self, n):
 
             sage: G = graphs.CycleGraph(3)
             sage: H = G * 3; H
-            Cycle graph disjoint_union Cycle graph disjoint_union Cycle graph: Graph on 9 vertices
+            Disjoint union of 3 copies of Cycle graph: Graph on 9 vertices
             sage: H.vertices(sort=True)
             [0, 1, 2, 3, 4, 5, 6, 7, 8]
             sage: H = G * 1; H
             Cycle graph: Graph on 3 vertices
+
+        TESTS::
+
+            sage: Graph(1) * -1
+            Traceback (most recent call last):
+            ...
+            TypeError: multiplication of a graph and a nonpositive integer is not defined
+            sage: Graph(1) * 2.5
+            Traceback (most recent call last):
+            ...
+            TypeError: multiplication of a graph and something other than an integer is not defined
         """
         if isinstance(n, (int, Integer)):
             if n < 1:
                 raise TypeError('multiplication of a graph and a nonpositive integer is not defined')
             if n == 1:
                 return copy(self)
-            return sum([self] * (n - 1), self)
-        else:
-            raise TypeError('multiplication of a graph and something other than an integer is not defined')
+            ns = self.order()
+            ntot = n * ns
+            vint = {u: i for i, u in enumerate(self)}
+            edges = ((i, j, l) for u, v, l in self.edge_iterator()
+                     for i, j in zip(range(vint[u], ntot, ns),
+                                     range(vint[v], ntot, ns)))
+            return self.__class__([range(ntot), edges], format='vertices_and_edges',
+                                  loops=self.allows_loops(),
+                                  multiedges=self.allows_multiple_edges(),
+                                  immutable=self.is_immutable(),
+                                  name=f"Disjoint union of {n} copies of {str(self)}")
+        raise TypeError('multiplication of a graph and something other than an integer is not defined')
 
     def __ne__(self, other):
         """
@@ -809,7 +829,7 @@ def __rmul__(self, n):
 
             sage: G = graphs.CycleGraph(3)
             sage: H = int(3) * G; H
-            Cycle graph disjoint_union Cycle graph disjoint_union Cycle graph: Graph on 9 vertices
+            Disjoint union of 3 copies of Cycle graph: Graph on 9 vertices
             sage: H.vertices(sort=True)
             [0, 1, 2, 3, 4, 5, 6, 7, 8]
         """