Upgrade cargo independently

[brson]

There may be good reason to upgrade Cargo beyond the version that comes with Rust.

[brson]

cc #25

[huonw]

This would be enable one to more easily use cargo with a --link-local'd rustc which doesn't seem to have cargo by default?

[brson]

I'm not sure about that yet. Because --link-local just makes a symlink to some random directory, to install some other Cargo on top of that would probably mean putting Cargo into some special location, not just installing into the normal toolchain directory.

[brson]

Here's what I want to do.

First, change the .multirust/toolchains/{toolchain} directory to not directly contain the installation, because we need more room for multirust-specific metadata (#30).

Next, add a subdirectory .multirust/toolchain/{toolchain}/aux-install to hold another installation tree.

Next, add multirust update {toolchain} --add-cargo {date|tarball} to install the specified cargo to aux-install. Update multirustproxy to configure the environment correctly and call the correct bin.