Use gofumpt

[Bartek Pacia]

gofumpt is a stricter, drop-in replacement for gofmt.

Personally, I've been using it for a few years and I like that it takes even more formatting decisions from me, so I don't focus on them.

I think it'd be cool it this project used it. The code formatting style would be more unified, which is useful since it's a popular project and many different people contribute to it.