Improve the Interface trait

[rylev]

Note: since this involves changes to generated code, it's easiest to review this commit by commit. The codegen was run in a separate commit from the other changes.

This improves the safety auditability of the Interface trait:

• Adds safety docs to the Interface trait. Even though this trait is not meant for public consumption, the docs can really help those reviewing the safety of generated code.
• Adds an as_raw method to the trait which simply wraps std::mem::transmute_copy. transmute_copy is very powerful and easy to use wrong. Having a bespoke method for converting an interface to a raw pointer which is documented as such makes the code easier to read. We've also had several users ask for this type of functionality before.
• Unmarks Interface::vtable as unsafe since this method is in fact safe.

Finally, we use the Interface::as_raw method instead of std::mem::transmute_copy in the generated code.

cc @eholk

[kennykerr]

Adds safety docs to the Interface trait. Even though this trait is not meant for public consumption, the docs can really help those reviewing the safety of generated code.

Note that parts of this trait are meant for public consumption. That's why some members were hidden. Notably, IID and cast are meant to be used by developers. The rest should remain hidden.

[rylev]

@kennykerr that sounds fine. I would argue for the newly added as_raw to also be publicly accessible as I think it's a better API (with many parallels in the standard library) than having to use std::mem::transmute_copy.

[rylev]

@kennykerr the latest nightly of clippy brought some new things to fix which I did, but it made the reviews stale. Can you re-review (only the last commit is new), and merge?