Cython speedup for CoordinateSequence.__iter__

This commit adds a Cython speedup for the `__iter__` method on `shapely.coords.CoordinateSequence`.

To test I've been using the code from issue https://github.com/Toblerity/Shapely/issues/93.

Without speedups, 500 iterations takes 31 seconds on my machine. With the existing speedups this is reduced to 6 seconds. This commit takes this down to less than 1 second.