Add KANN, a deep learning library in C

[attractivechaos]

KANN is like a CPU-only mini-tensorflow, providing common components for deep learning. KANN implements automatic differentiation (autodiff; the foundation of general deep learning frameworks) in two files kautodiff.{h,c} and implements deep learning utilities in two additional files. So, if you need autodiff only, take two files; if you need deep learning, take all four files.

I am fully aware that the rule is "libraries should use at most two files", but I also notice "exceptions will be allowed for good reasons". In case of KANN, "good reasons" include 1) autodiff is logically separated from deep learning and is more general; 2) the autodiff part of KANN is qualified for a two-file library; 3) KANN is already the smallest of its kind with most other frameworks many times larger.

I hope you may consider KANN as an exception, but if you prefer to reject this PR, I also fully understand. Thanks in advance.