Make the generators of CoxeterGroup dense.

The one() element is dense because the matrix_space() is
dense. By making the generators dense, it means the generators
are not converted to dense matrices upon multiplication,
resulting in a 2x speedup to iteration.

src/sage/groups/matrix_gps/coxeter_group.py

@@ -266,6 +266,8 @@ def __init__(self, coxeter_matrix, base_ring, index_set):
                                            for j in range(n)},
                               coerce=True, copy=True)
                 for i in range(n)]
+        # Make the generators dense matrices for consistancy and speed
+        gens = [g.dense_matrix() for g in gens]
         category = CoxeterGroups()
         # Now we shall see if the group is finite, and, if so, refine
         # the category to ``category.Finite()``. Otherwise the group is