Add vec!['\0'; n] optimization, like vec![0; n]

Similarly to vec![ptr::null{,_mut}(); n] in previous change, this adds
the optimization for vec!['\0'; n].

src/liballoc/vec.rs

@@ -1611,6 +1611,8 @@ fn is_zero(&self) -> bool {
 impl_is_zero!(u128, |x| x == 0);
 impl_is_zero!(usize, |x| x == 0);
 
+impl_is_zero!(char, |x| x == '\0');
+
 impl_is_zero!(f32, |x: f32| x.to_bits() == 0);
 impl_is_zero!(f64, |x: f64| x.to_bits() == 0);