Alpha v0.4.0: add go.mod and go.sum

I'm not creating a branch to lock dependency versions during pre-1.0 development.  Instead,
I'll create orphaned commits for each alpha and beta release, adding go.mod and go.sum to
those and freeze dependency versions each time.

This way, I'm still supporting go modules kind of, without locking myself to older dependency
versions for prerelease development.