Choose or write a style guide for Idris code and adopt it

[yurrriq]

e.g. https://github.com/david-christiansen/idris-code-highlighter/blob/master/CONTRIBUTING.md

[clayrat]

Looks reasonable except for the explicit implementation, I find it too verbose :)

[yurrriq]

I'm happy to concede on that point. Feel free to revert my insertions on implementation in develop or you want.

[yurrriq]

Let's make our our style guide then, perhaps forming that one and removing the implementation part.