Convert recursive method into iterative to prevent function calls

[berendkleinhaneveld]

Just after merging #142 I thought of making this method non-recursive to prevent the overhead of the recursive function call. This should improve the performance a bit more, although not as dramatically as #142.

[berendkleinhaneveld]

------------------------ benchmark 'traverse_deep': 2 tests ------------------------
Name (time in us)                          Mean             StdDev            Rounds
------------------------------------------------------------------------------------
test_traverse_deep (NOW)               899.7457 (1.0)      27.3784 (1.0)         712
test_traverse_deep (0001_master)     1,161.3173 (1.29)     59.9269 (2.19)        593
------------------------------------------------------------------------------------

Not too bad!