Exec the file instead of using cargo run

[fornwall]

This change makes rust-script execute the built binary directly (if it's newer than the generated Cargo.toml and main.rs files).

• Speeds things up: Excessive script overhead #39
• Avoid the build being affected by rust toolchain files, since the build is executed inside of the generated package directory instead of the current working directory: rust-toolchain affects which rustc version is used to execute a script #31
• Avoids always running cargo, making -o output verbose: expose a way to show compilation progress? #30 (specifically this comment).

Let's drop --features (it can be added back if people are using it - it's just necessary to --force a rebuild if changing the features).