Generate completion files on toolchain switch or update

Problem you are trying to solve

To utilize rustup completions, you are suggested to run rustup completions every time login shell is opened https://rust-lang.github.io/rustup/installation/index.html#enable-tab-completion-for-bash-fish-zsh-or-powershell. On some machines, this may drastically affect shell startup time

Solution you'd like

rustup completions can be called in a rustup internal post-install routine and generate proper completion files after updating or switching toolchain (or at first install); users will be able to source pre-baked completion files without calling rustup each time they login to shell

Notes

An example of how this process may look for zsh:

• calling rustup update alters rustup and cargo versions, new cli arguments are added
• after successful update, a post-install routine of completion generation started:
  • rustup completions zsh rustup >$RUSTUP_HOME/completions/_rustup makes completions for rustup
  • rustup completions zsh cargo >$CARGO_HOME/completions/_cargo makes completions for cargo
  • performing rm -f ~/.zcompdump; compinit to reset and reload completions in zsh
• if user has FPATH="$RUSTUP_HOME/completions:$CARGO_HOME/completions:$FPATH" in their .zshrc, the completions will work and be up-to-date

Start with the rustup completions, rustup update, and toolchain-switch entry points, and compare the installation guide's shell setup. Trace when post-install generation would run and where RUSTUP_HOME and CARGO_HOME completion files belong. Done means completions are regenerated after install, update, or switch without invoking rustup at shell startup.