Use rust-installer for installation

This factors out the install.sh file used by both Cargo and Rust in anticipation of creating a single combined installer for both, per https://github.com/rust-lang/rust/issues/16456.

There should be no changes in functionality.

The installation script has been moved to [a new shared project](https://github.com/rust-lang/rust-installer).

The impacts to Cargo are:
  * This adds a submodule that builders will need to initialize. It doesn't add any sort of auto-initialization in `configure` like Rust uses, so the buildbots will need to change (or `configure` will need to).
  * When running `make install` or `make dist`, `rust-installer` itself does yet-another copy of the installation image during its prep.
  * When running `make install` the tarball is built as a side-effect, though it isn't used.

We might want to wait a week to land this since I will be OOTO from Wednesday through Sunday and unable to deal with fallout.