Switch to DSTs

[phip1611]

After this was discussed in #118 (comment) and #128 (comment), I came to the conclusion, that all such structs should be refactored to DSTs. This will satisfy miri and also cleans up code.

[phip1611]

The feature in Rust is unstable, but luckily, there is https://crates.io/crates/ptr_meta

[YtvwlD]

oh, I've done this in #133 :)

[phip1611]

Oh, I did not see this at a first glance. Now we also have #134 😬