Optimise vec![false; N] to zero-alloc

Nowadays booleans have a well-defined representation, so there is no reason not to optimise their allocation.

Author: RReverser

src/liballoc/vec.rs

@@ -1606,6 +1606,7 @@ impl_is_zero!(u64, |x| x == 0);
 impl_is_zero!(u128, |x| x == 0);
 impl_is_zero!(usize, |x| x == 0);
 
+impl_is_zero!(bool, |x| x == false);
 impl_is_zero!(char, |x| x == '\0');
 
 impl_is_zero!(f32, |x: f32| x.to_bits() == 0);