Ability to use `SliceWithHeader` in a newtype directly without `Box`

[ggutoski]

I want to make a pair of newtypes MySlice(SliceWithHeader<HeaderType, u8>) and MyVec(HeaderType, Vec<u8>), so that MySlice is to MyVec as [u8] is to Vec<u8>.

It seems as per #69 that this crate offers only the ability to make MySlice(Box<SliceWithHeader<...>>) with Box (or Rc, Arc) but not without the Box.

I know it's possible to do this without the header as per How can I create newtypes for an unsized type and its owned counterpart (like str and String) in safe Rust? - Stack Overflow. But it's not clear to me how to get both (i) no Box, and (ii) header. Any suggestion?