message: Improve Starlark/Rust interop example
  Summary: Makes example easier to follow (I hope).
diff: //! use starlark::syntax::{AstModule, Dialect};
  //! use starlark::values::Value;
  //!
  -//! let content = r#"
  -//! quadratic(4, 2, 1, x = 8)
  -//! "#;
  -//!
  +//! // This defines the function that is visible to Starlark
  //! #[starlark_module]
  //! fn starlark_quadratic(builder: &mut GlobalsBuilder) {
  -//! // This defines a function that is visible to Starlark
  //! fn quadratic(a: i32, b: i32, c: i32, x: i32) -> i32 {
  //! Ok(a * x * x + b * x + c)
  //! }
  //! }
  //!
  -//! let ast = AstModule::parse("quadratic.star", content.to_owned(), &Dialect::Standard)?;
  -//! // We build our globals adding some functions we wrote
  +//! // We build our globals to make the function available in Starlark
  //! let globals = GlobalsBuilder::new().with(starlark_quadratic).build();
  //! let module = Module::new();
  //! let mut eval = Evaluator::new(&module, &globals);
  +//!
  +//! // Let's test calling the function from Starlark code
  +//! let starlark_code = r#"
  +//! quadratic(4, 2, 1, x = 8)
  +//! "#;
  +//!
  +//! let ast = AstModule::parse("quadratic.star", starlark_code.to_owned(), &Dialect::Standard)?;
  //! let res = eval.eval_module(ast)?;
  -//! assert_eq!(res.unpack_int(), Some(273));
  +//! assert_eq!(res.unpack_int(), Some(273)); // Verify that we got an `int` return value of 4 * 8^2 + 2 * 8 + 1 = 273
  //! # Ok(())
  //! # }
  //! # fn main(){ run().unwrap(); }
  