all: run gofumpt (#652)

* all: run gofumpt

https://github.com/mvdan/gofumpt

The main appeal for me here is that my (and many other peoples')
editors run this on save, so doing a single diff here
keeps other diffs minimal.

* .github/workflows: add gofumpt checker

plugin/simplemath/main.go

@@ -13,7 +13,6 @@ import (
 )
 
 func main() {
-
 	libplugin.CreateAndRunPluginTemplate(&libplugin.PluginTemplate{
 		Name:  "simplemath",
 		Usage: "sshpiperd simplemath plugin, do math before ssh login",