Support unsized structs

[overlookmotel]

It is currently not possible to derive Inspect on unsized structs.

Should be possible to at least determine alignment with a derive macro by calculating max of alignment of field types or parsing #[repr(align(...))] attribute.

However, calculating offset of fields is not possible (Gilnaa/memoffset#25). Native Rust offset_of! macro may help somewhat, but still isn't a complete solution.

Some initial work on unsized branch.