Optimise reversed()

[scoder]

On iteration, the builtin reversed() function can be reinterpreted as a hint to the looping optimiser in many cases, in order to generate well adapted looping code instead of generic Python iteration code. A good example is reversed(list/tuple) where the fast indexing loop can simply start from the end.

[scoder]

Implemented here:

 https://github.com/cython/cython/commit/c7e5af8b3979668d6f40b76b7ae26e382e6410eb

and in the subsequent commits up to

 https://github.com/cython/cython/commit/09b4c92ab231caaf00237e0de7c311e9d8bf423a