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

[jxcl]

r? @steveklabnik

Fixes issue #22518.

(I accidentally pushed another commit onto this PR, so I've closed and reopened it.)

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.

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[steveklabnik]

@jxcl while I appreciate this, I'm not ready to move forward on it yet. rand may come back in enough form by 1.0 that we don't need to make this churn. If it doesn't, we should start from this patch, though. I'm going to give this a close until we figure out the schedule.