Skip generated impls using Erased attribute.

[maximebuyse]

Fixes #1070

Instead of just skipping generated impls, this adds the Erased attribute to them so that the body is dropped but they still exist in the engine. This allows for them to be moved by bundling thus removing the spurious let open.

[maximebuyse]

Two comments here:

• the skip attribute seems to have been removed so maybe we can remove the corresponding logic in import_thir?
• there is a coq test that fails because of this (it generates a type class instance with dummy methods), maybe this means we should also implement the Erased attribute in the other backends.

[maximebuyse]

Two comments here:

* the `skip` attribute seems to have been removed so maybe we can remove the corresponding logic in `import_thir`?

* there is a coq test that fails because of this (it generates a type class instance with dummy methods), maybe this means we should also implement the `Erased` attribute in the other backends.

For coq, I added a quick fix: it filters out trait impls that have the Erased attribute. I opened #1185 to add full support for Erased in the coq backend.

[maximebuyse]

Two comments here:

* the `skip` attribute seems to have been removed so maybe we can remove the corresponding logic in `import_thir`?

* there is a coq test that fails because of this (it generates a type class instance with dummy methods), maybe this means we should also implement the `Erased` attribute in the other backends.

For skip we discussed it and it is indeed unused so I removed the logic. @W95Psp You can finish your review, I am done with my changes.

[W95Psp]

Looks great, thanks!