[Cython] Please vote: wraparound directive

[Dag Sverre Seljebotn]

A new directive which allows Cython to assume that integers passed to [] 
are not negative.

Currently buffers have the negative_indices option:

   object[int, negative_indices=False] arr = ...

but this can be cumbersome and less flexible. The rationale for not 
doing this in the first place was that it changes semantics during 
normal running rather than under exceptional circumstances, but now 
@cdivision has crossed that line anyway.

Also note that this could potentially apply to typed lists and tuples as 
well, if there's a benefit.

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Dag Sverre