gup updates and manages the global Go command-line tools in $GOBIN.
`go install` places each program there but never updates it again, keeps no
manifest of what it installed, and offers no way to hold a tool at a
version you depend on. gup brings the whole set up to date in parallel,
pins selected tools to exact versions, and adds the management commands
`go install` lacks: list and check what is installed, remove binaries,
export and import the set to reproduce it on another machine, and migrate
it to a new $GOBIN.
