In-depth investigation of slowness #190
Compiled with release using the benchmark codes:
[Arraymancer tensor](https://github.com/mratsim/Arraymancer/blob/4d93ed4bec63d909cf2e6f2e7041fa21048cb619/benchmarks/euler_tensor.nim) 23.32s.
[Native array](https://github.com/mratsim/Arraymancer/blob/4d93ed4bec63d909cf2e6f2e7041fa21048cb619/benchmarks/euler_nativearray.nim) 0.85s.
[Numpy](https://github.com/mratsim/Arraymancer/blob/4d93ed4bec63d909cf2e6f2e7041fa21048cb619/benchmarks/euler_numpy.py) 6.28s

on a `i5-5257U (2.7GHz dual core mobile Broadwell)`

![2018-03-17_11-03-02](https://user-images.githubusercontent.com/22738317/37554028-ef8fe1b8-29d2-11e8-9d7a-0c668e325e49.png)

linked to #190