Reworked guessing-game to use the random number crate found at crates.io...

[jxcl]

r? @steveklabnik

Fixes issue #22518.

Notes:

• Removed discussion of standard library from guessing game in the context of
  std::rand because rand is no longer part of the standard library. Maybe it
  can be put back in in the discussion of I/O?
• The compiler is now able to infer the return type of rand::random() without
  any type annotations.
  let secret_number = (rand::random() % 100) + 1; // secret_number: i32

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @steveklabnik (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.