message: Make CodeMap for Starlark only
  Summary: Before we had introductory docs on how to use it, and a creation function. Remove that. Now you can't use CodeMap for your own things, only Starlark stuff, which is how we'd prefer it was used.
diff: //! 32-bit `Pos` indexing into the `CodeMap`, under the assumption that the total amount of parsed
  //! source code will not exceed 4GiB. The `CodeMap` can look up the source file, line, and column
  //! of a `Pos` or `Span`, as well as provide source code snippets for error reporting.
  -//!
  -//! # Example
  -//! ```
  -//! use starlark::codemap::{CodeMap, Pos, Span};
  -//! let mut codemap = CodeMap::new("test.rs".to_owned(), "fn test(){\n println!(\"Hello\");\n}\n".to_owned());
  -//! let string_literal_span = Span::new(Pos::new(24), Pos::new(31));
  -//!
  -//! let location = codemap.look_up_span(string_literal_span);
  -//! assert_eq!(location.file.filename(), "test.rs");
  -//! assert_eq!(location.begin.line, 1);
  -//! assert_eq!(location.begin.column, 13);
  -//! assert_eq!(location.end.line, 1);
  -//! assert_eq!(location.end.column, 20);
  -//! ```
  use gazebo::prelude::*;
  use std::{
  cmp,
  @@ -173,7 +159,7 @@ impl Eq for CodeMap {}
  impl CodeMap {
  /// Creates an new `CodeMap`.
  - pub fn new(filename: String, source: String) -> CodeMap {
  + pub(crate) fn new(filename: String, source: String) -> CodeMap {
  let mut lines = vec![Pos(0)];
  lines.extend(source.match_indices('\n').map(|(p, _)| Pos(p as u32 + 1)));
  