Install zsh completions for core tools

[ghost]

Everything below is about zsh. I have no idea about bash.

Both cargo and rustc have completions: here and here not even speaking of rustup, that can generate its own ones. It took me a good amount of time to find these completions for every project and create them manually. I guess they could and should be installed alongside with the tools, in particular:

• ask whether the user wants to install completions for core tools
• if yes, ask whether it should be system-wide or local
• if system-wide, install to /usr/share/zsh/site-functions, if local, install somewhere in inner directories (~/.multirust/completions maybe? not sure) and tell the user to append this path to $fpath variable before compinit is executed

Also, I'm talking about the bootstrapper from the website ... | sh. Other types of installations might already do this. But in case they don't, it'd nice to have it there too.

Also, I'm not sure that rustc should have completions command - this is "one-and-done" and is useless afterwards only stealing attention.

Also, not sure about the debugger and other tools - they may need completions as well.

[Diggsey]

Dupe of #387