std: Remove Zero impl for Option

Options are not numeric types, so it doesn't make sense for them to
implement Zero.

Author: erickt

src/libstd/option.rs

@@ -483,11 +483,6 @@ impl<T:Zero> Option<T> {
     }
 }
 
-impl<T> Zero for Option<T> {
-    fn zero() -> Option<T> { None }
-    fn is_zero(&self) -> bool { self.is_none() }
-}
-
 /// An iterator that yields either one or zero elements
 #[deriving(Clone, DeepClone)]
 pub struct OptionIterator<A> {

src/test/run-pass/deriving-zero.rs

@@ -24,7 +24,6 @@ struct E { a: int, b: int }
 
 #[deriving(Zero)]
 struct Lots {
-    c: Option<util::NonCopyable>,
     d: u8,
     e: char,
     f: float,