Rename Div operator trait to Quot and Modulo operator trait to Rem

This renaming, proposed in the [Numeric Bikeshed](https://github.com/mozilla/rust/wiki/Bikeshed-Numeric-Traits#rename-modulo-into-rem-or-remainder-in-traits-and-docs), will allow us to implement div and and modulo methods that follow the conventional mathematical definitions for negative numbers without altering the definitions of the operators (and confusing systems programmers). Here is a useful answer on StackOverflow that explains the difference between `div`/`mod` and `quot`/`rem` in Haskell: (When is the difference between quotRem and divMod useful?)[http://stackoverflow.com/a/339823/679485].

This is part of the numeric trait reforms tracked in issue #4819.