Remove GoPackage.Standard field since it's no longer used.
Rename ".../gists/gist5892738" package to ".../trim".

You might have to run `go get -u` twice, sorry about that. I've opened
issue golang/go#9224 and hope it will be resolved someday.
According to
godoc.org/github.com/shurcooL/go/gists/gist5892738?importers there are
no other users.
Make the previously manual test into an automated one.