loop unrolling of one of the core numpy_iexpr loop

One of the critical loop of numpy_iexpr was not unrolled by GCC. In the
new (templated) form, it does, which leads to some performance
improvements.

On the numpy-benchmarks test suite, we never get a slowdown and sometime
get a 10% speedup compared to master.
Merge pull request #264 from serge-sans-paille/faster-numpy-iexpr

Force loop unrolling of one of the core numpy_iexpr loop