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 + 1 // wrapping_neg
             } 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) + 1; // wrapping_neg
 
                 // Make sure we didn't underflow.
                 if res > 0 {