structlayout: improve the CLI

[dominikh]

The CLI of the various structlayout tools needs to be improved. To summarize the open issues from the old repository:

• structlayout panics when being passed the wrong arguments
• the tools don't complain about unused arguments
• structlayout is invoked with structlayout <importpath> <type name>, which works but isn't great for usability. We should additionally support guru's format of specifying a file and position.
• the help output of the various tools could be improved.