Change the representation of str to be able to compile more transmute casts

[protz]

@ssyram this is a WIP -- I tried to extract more code from the str module

I think in the long run, we will want a mechanism to encode some type equations for when things have the same representation, like you have done on your branch -- this is one case where we have to know that str is a slice of u8, and it looks based on your branch that there are more such cases

[ssyram]

Ah, yes, there are many such cases, e.g., in tmp-checker branch, I added a function essentially_slice in the MonoState module, which is:

let essentially_slice lid =
  let (|>) x f = f x in
  [
    [ "std"; "path" ], "Path";
    [ "std"; "ffi"; "os_str" ], "OsStr";
  ]
  |> List.mem lid

But I don't think we should handle them by hand by ourselves, ultimately, after the mono migration, we will be able to know their metadata and translate as Eurydice::dst_ref -- this is what is happening now.

The problem now is just that in the generic case like fn by_ref<T>(t: &T) { ... } or so, the type checker cannot recognise these types (e.g., std::path::Path) as DST. But they will not appear in mono mode.

This is how I understand it, but might be wrong, would you have a deeper explanation to your intention if this is not what you mean?