Rollup merge of #106854 - steffahn:drop_linear_arc_rebased, r=Mark-Simulacrum

Rollup merge of #106854 - steffahn:drop_linear_arc_rebased, r=Mark-Simulacrum

Add `Arc::into_inner` for safely discarding `Arc`s without calling the destructor on the inner type.

ACP: rust-lang/libs-team#162

Reviving #79665.

I want to get this merged this time; this does not contain changes (apart from very minor changes in comments/docs).

See #79665 for further description of the PR. The only “unresolved” points that led to that PR being closed, AFAICT, were

* The desire to also implement a `Rc::into_inner` function
  * however, this can very well also happen as a subsequent PR
* Possible need for further discussion on the naming “`into_inner`” (?)
  * `into_inner` seems fine to me; also, this PR introduces unstable API, and names can be changed later, too
* ~~I don't know if a tracking issue for the feature flag is supposed to be opened before or after this PR gets merged (if *before*, then I can add the issue number to the `#[unstable…]` attribute)~~ There is a [tracking issue](https://github.com/rust-lang/rust/issues/106894) now.

I say “unresolved” in quotation marks because from my point of view, if reviewers agree, the PR can be merged immediately and as-is :-)