Rollup merge of #22256 - brson:installer-next, r=alexcrichton
Highlights:
* Adds an 'uninstall.sh' script to `/usr/local/lib/rustlib/uninstall.sh`, the path to which is printed during installation.
* Components can be deselected during install, like `install.sh --without=rust-docs`.
* Components can be listed with `install.sh --list-components`.
* Vastly reduces spew during install (but supporting a `--verbose` option).