Friendlier compilation progress

[rfdonnelly]

I would like to use rust-script to implement scripts that others will run. Most will not be familiar with Rust let alone rust-script. The default behavior makes it look like the script is hung when compilation is being performed silently. An alternative is to use the --cargo-output option but this is too much output and too foreign for those not familiar with Rust/rust-script.

It would be nice if rust-script could be made (via an option) to print a messages like "Performing an initial one-time compilation..." and "Performing a recompilation..." or similar (maybe just "Building...") to indicate that something is happening.

It would be even nicer if the cargo progress bar could be shown w/o all of the other cargo output. But maybe this needs support from cargo.