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).
Typicall install run looks like:
```
brian@brianX1:~/dev/multirust⟫ sudo ./install.sh
[sudo] password for brian:
install: creating uninstall script at /usr/local/lib/rustlib/uninstall.sh
install: installing component 'rustc'
install: installing component 'cargo'
install: installing component 'rust-docs'
Rust is ready to roll.
```
Needs to be merged right before corresponding PRs to cargo and rust-packaging.
Fixes https://github.com/rust-lang/rust/issues/21117
Fixes https://github.com/rust-lang/rust/issues/20283
97503e1c1e