Add experimental setup

Not all tests are shown in the paper, but Pythran performs well on them.
Comment experiments & benchmarks.
Fix missing declaration for zeros_like implementation with boost.simd
Take into account other reviews.
Improve slicing by making the step field a constant when possible

It appears that slices often have a step of one, which turns them into
contiguous slices. Many optimizations can be performed on contiguous
slices, so take advantage of it!