Option<SmallVecN<T>> breaks the non-zero optimization

[SimonSapin]

This currently fails:

assert!(Some(SmallVec4::<&u32>::new()).is_some());

This is because SmallVec<&u32> contains a [&u32; 4] field directly, and the compiler choses to use one of those &u32 (which are non-zero) to set to zero to represent None in Option<SmallVec<&u32>>. However, new initializes the array with std::mem::zeroed(), which conflicts with this optimization.

https://github.com/bluss/arrayvec works around this by using #[repr(u8)] on an enum. (The one used to make drop sound, see #4.)

CC @pcwalton