Streamline install to a single command.

[JoshuaBatty]

From Nick on Slack.

Can we incorporate Rust installation into Fuelup?

Ideally, a developer can just run curl --proto '=https' --tlsv1.2 -sSf https://install.fuel.network/fuelup-init.sh | sh Which should give them the latest toolchain, but before that, it should ask "Would you like to install rust?" if it detects that Rust is not installed.

It can then suggest to run the following command if Rust is not installed: curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs/ | sh

The end goal should be reducing this down to a single command, and not what we have now which is 4 commands to install: