move from non zero impls to libcore/convert/num.rs

Author: Peter

CONTRIBUTING.md

@@ -150,13 +150,13 @@ All pull requests are reviewed by another person. We have a bot,
 request.
 
 If you want to request that a specific person reviews your pull request,
-you can add an `r?` to the message. For example, [Steve][steveklabnik] usually reviews
+you can add an `r?` to the pull request description. For example, [Steve][steveklabnik] usually reviews
 documentation changes. So if you were to make a documentation change, add
 
     r? @steveklabnik
 
-to the end of the message, and @rust-highfive will assign [@steveklabnik][steveklabnik] instead
-of a random person. This is entirely optional.
+to the end of the pull request description, and [@rust-highfive][rust-highfive] will assign
+[@steveklabnik][steveklabnik] instead of a random person. This is entirely optional.
 
 After someone has reviewed your pull request, they will leave an annotation
 on the pull request with an `r+`. It will look something like this: