Format all code according to `gofmt` standard, add Makefile rule

The Go language and its community strongly encourage you to format your code according to the One True Style — indent using tab characters, opening brace on the same line as the conditional, etc. The language’s tooling makes it very easy to do this: go fmt ./... will rewrite all your project’s source files in place.

I’ve resisted doing this because there are some things about the standard formatting that I dislike: most important, that it prohibits multiple blank lines between functions. But I’ve come to the conclusion that adhering to the standard is more important than preserving this preference. The value proposition of gofmt is “never think about formatting again”, and I think that trade-off is worth making.