PSA: Rust documentation is now optional and opt-in when installing via rustup

[brson]

@tcf Yes, that would be better.

The point that docs are a only small part of the download time is well taken.

I'm still undecided about what to do here but it seems like it's going to be a combination of multiple things. Here's what I'm inclined to do:

• Modify the mainifests (rust-buildbot) so that rust-docs are required again.
• Optimize rustup's installation routine so it isn't moving around so much data. There's a lot of temporary files involved and a lot of improvements possible. The actual on-disk installation can go a lot faster.
• Teach rustup and the rust-buildbot manifest generator about optional but installed by default components
• Modify the manifests so that rust-docs is optional but installed by default
• Add a --minimal flag to rust-init that gives you a working rustc+cargo but no optional components, for use by e.g. automation.
• Add command-line and interactive component selection to rust-init.

Thanks for your patience.