Neural Net DSL overhead
#214 introduced a domain specific language for neural net.

It seems like it has a noticeable overhead, at least on a simple perceptron.

benchmark on XOR https://github.com/mratsim/Arraymancer/commit/c19b2050622f5d1ef1ec9a04e1e34f041dc7a59c

with Nim devel from 2018-05-06 (commit hash not displayed for some reason :/)

![2018-05-09_22-19-12](https://user-images.githubusercontent.com/22738317/39837888-fc784c7e-53d7-11e8-8a8f-2194c2d5f38c.png)
![2018-05-09_22-20-53](https://user-images.githubusercontent.com/22738317/39837891-fea56d10-53d7-11e8-81a3-51fe699a033f.png)
