message: Make the docs use the Display/collect_repr approach
  Summary: Move the docs to the new standard approach for doing things.
diff: //! use starlark::syntax::{AstModule, Dialect};
  //! use starlark::values::{Heap, SimpleValue, StarlarkValue, Value, ValueError, ValueLike};
  //! use starlark::{starlark_type, starlark_simple_value};
  +//! use std::fmt::{self, Display, Write};
  //!
  //! // Define complex numbers
  //! #[derive(Debug, PartialEq, Eq)]
  //! }
  //! starlark_simple_value!(Complex);
  //!
  +//! impl Display for Complex {
  +//! fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  +//! write!(f, "{} + {}i", self.real, self.imaginary)
  +//! }
  +//! }
  +//!
  //! impl<'v> StarlarkValue<'v> for Complex {
  //! starlark_type!("complex");
  //!
  //! // How we display them
  //! fn collect_repr(&self, collector: &mut String) {
  -//! collector.push_str(&format!("{} + {}i", self.real, self.imaginary))
  +//! write!(collector, "{}", self).unwrap()
  //! }
  //!
  //! // How we add them
  