Add nnpack as a git submodule
Nnpack does not have any release and is only available through git cloning. This means that macOS and most Linux distributions do not package it.

Thankfully `nimble` pulls the git submodule when installing a Nim package. It would be great to have [nnpack](https://github.com/Maratyszcza/NNPACK) auto-pulled with Arraymancer and use the `{.compile.}` pragma to compile it (the pragma works for whole directories) and link it to Arraymancer.

This would avoid the hassle of installing it manually.

A `third_party` or `external` folder should be added at the root as Arraymancer will probably pull multiple external libs in the future (OpenCL, ...)