Move trait impls for primitives near trait definition

Closes #12925

src/libcore/default.rs

@@ -16,10 +16,33 @@ pub trait Default {
     fn default() -> Self;
 }
 
-impl Default for () {
-    #[inline]
-    fn default() -> () { () }
-}
+macro_rules! default_impl(
+    ($t:ty, $v:expr) => {
+        impl Default for $t {
+            #[inline]
+            fn default() -> $t { $v }
+        }
+    }
+)
+
+default_impl!((), ())
+default_impl!(bool, false)
+default_impl!(char, '\x00')
+
+default_impl!(uint, 0u)
+default_impl!(u8,  0u8)
+default_impl!(u16, 0u16)
+default_impl!(u32, 0u32)
+default_impl!(u64, 0u64)
+
+default_impl!(int, 0i)
+default_impl!(i8,  0i8)
+default_impl!(i16, 0i16)
+default_impl!(i32, 0i32)
+default_impl!(i64, 0i64)
+
+default_impl!(f32, 0.0f32)
+default_impl!(f64, 0.0f64)
 
 impl<T: Default + 'static> Default for @T {
     fn default() -> @T { @Default::default() }