Fix fallout from wrapping_neg revert

Author: brson

src/libcore/num/mod.rs

@@ -506,7 +506,7 @@ macro_rules! int_impl {
         #[inline]
         pub fn abs(self) -> $T {
             if self.is_negative() {
-                self.wrapping_neg()
+                -self
             } else {
                 self
             }

src/libserialize/json.rs

@@ -1538,7 +1538,7 @@ impl<T: Iterator<Item=char>> Parser<T> {
             F64Value(res)
         } else {
             if neg {
-                let res = (res as i64).wrapping_neg();
+                let res = -(res as i64);
 
                 // Make sure we didn't underflow.
                 if res > 0 {