libc::Nullable should have attribute that enforces null-pointer optimization

[lilyball]

libc::Nullable exists as an Option-equivalent that is suitable for using with liblibc without requiring libstd. It seems the appropriate type to use for nullable function pointers, e.g. Nullable<extern "C" fn (int)>. But doing this is making the implicit assumption that Nullable benefits from the null-pointer optimization, i.e. that this type has the same size as *(). I think we should statically guarantee this by adding an attribute that guarantees that Nullable<T> can only be used with a type parameter that allows it to use the null-pointer optimization. This means I can say Nullable<extern "C" fn (int)>, but I can't say Nullable<uint>, nor Nullable<*u32> (because *u32 is already nullable, and therefore the null-pointer optimization doesn't apply).

[alexcrichton]

This type no longer exists, it's now Option, so closing.