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: https://github.com/fornwall/rust-script/issues/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: https://github.com/fornwall/rust-script/issues/31
• Avoids always running cargo, making -o output verbose: https://github.com/fornwall/rust-script/issues/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).