[WIP] Stdlib cleanup

The stdlib is a bit messy at the moment. Let's rework it to make it more streamlined.

For example: `layout ruby` and `layout python` both enable local package repositories. Not all users want that.

The `use` command dispatch doesn't have a great story behind it's usage. How are people supposed to install packages in /usr/local/Cellar ? Plus homebrew doesn't really like multiple version and will remove them when running `brew cleanup`

(previously issue #55)