message: Add a demo of repeatedly calling a function
diff: //! # }
  //! # fn main(){ run().unwrap(); }
  //! ```
  +//!
  +//! ## Call a Starlark function from Rust
  +//!
  +//! You can extract functions from Starlark, and call them from Rust, using `eval_function`.
  +//!
  +//! ```
  +//! # fn run() -> anyhow::Result<()> {
  +//! use starlark::environment::{Globals, Module};
  +//! use starlark::eval::Evaluator;
  +//! use starlark::syntax::{AstModule, Dialect};
  +//! use starlark::values::Value;
  +//!
  +//! let content = r#"
  +//! def quadratic(a, b, c, x):
  +//! return a*x*x + b*x + c
  +//! quadratic
  +//! "#;
  +//!
  +//! let ast = AstModule::parse("quadratic.star", content.to_owned(), &Dialect::Extended)?;
  +//! let globals = Globals::default();
  +//! let module = Module::new();
  +//! let mut eval = Evaluator::new(&module, &globals);
  +//! let quad = eval.eval_module(ast)?;
  +//! let res = eval.eval_function(
  +//! quad,
  +//! &[Value::new_int(4), Value::new_int(2), Value::new_int(1)],
  +//! &[("x", Value::new_int(8))],
  +//! )?;
  +//! assert_eq!(res.unpack_int(), Some(273));
  +//! # Ok(())
  +//! # }
  +//! # fn main(){ run().unwrap(); }
  +//! ```
  // Features we use
  #![feature(backtrace)]
  