ci: add golangci-lint

[mfridman]

Fix #453

This PR introduces golangci-lint to the project, which is run on every push to master/main and on every PR. The full documentation can be found here, and we can tune it as things come up.

If we don't like it, we'll remove it.

There are also two helpful make targets:

make tools
make lint

^ this should is effectively what CI will run.

The default linters are already pretty helpful:

errcheck
gosimple
govet
ineffassign
staticcheck
typecheck
unused