Make {Default, From, FromIterator, One, Zero} well-formed

Using these traits in an object context previously resulted in an RFC
1214 warning.

src/libcore/num/mod.rs

@@ -58,7 +58,7 @@ pub mod diy_float;
 #[unstable(feature = "zero_one",
            reason = "unsure of placement, wants to use associated constants",
            issue = "27739")]
-pub trait Zero {
+pub trait Zero: Sized {
     /// The "zero" (usually, additive identity) for this type.
     fn zero() -> Self;
 }
@@ -70,7 +70,7 @@ pub trait Zero {
 #[unstable(feature = "zero_one",
            reason = "unsure of placement, wants to use associated constants",
            issue = "27739")]
-pub trait One {
+pub trait One: Sized {
     /// The "one" (usually, multiplicative identity) for this type.
     fn one() -> Self;
 }