New trait OwnableRefCounted for ARef<->Owned conversion.

This allows to convert between ARef<T> and Owned<T> by
implementing the new trait OwnedRefCounted.

This way we will have a shared/unique reference counting scheme
for types with built-in refcounts in analogy to Arc/UniqueArc.

To: Miguel Ojeda <ojeda@kernel.org>
To: Alex Gaynor <alex.gaynor@gmail.com>
To: Boqun Feng <boqun.feng@gmail.com>
To: Gary Guo <gary@garyguo.net>
To: Björn Roy Baron <bjorn3_gh@protonmail.com>
To: Benno Lossin <benno.lossin@proton.me>
To: Andreas Hindborg <a.hindborg@kernel.org>
To: Alice Ryhl <aliceryhl@google.com>
To: Trevor Gross <tmgross@umich.edu>
To: Asahi Lina <lina@asahilina.net>
Cc: rust-for-linux@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Oliver Mangold <oliver.mangold@pm.me>
---
Changes in v8:
- EDITME: describe what is new in this series revision.
- EDITME: use bulletpoints and terse descriptions.
- Link to v7: https://lore.kernel.org/r/20250310-unique-ref-v7-0-4caddb78aa05@pm.me

Changes in v7:
- Squash patch to make Owned::from_raw/into_raw public into parent
- Added Signed-off-by to other people's commits
- Link to v6: https://lore.kernel.org/r/20250310-unique-ref-v6-0-1ff53558617e@pm.me

Changes in v6:
- Changed comments/formatting as suggested by Miguel Ojeda
- Included and used new config flag RUSTC_HAS_DO_NOT_RECOMMEND,
  thus no changes to types.rs will be needed when the attribute
  becomes available.
- Fixed commit message for Owned patch.
- Link to v5: https://lore.kernel.org/r/20250307-unique-ref-v5-0-bffeb633277e@pm.me

Changes in v5:
- Rebase the whole thing on top of the Ownable/Owned traits by Asahi Lina.
- Rename AlwaysRefCounted to RefCounted and make AlwaysRefCounted a
  marker trait instead to allow to obtain an ARef<T> from an &T,
  which (as Alice pointed out) is unsound when combined with UniqueRef/Owned.
- Change the Trait design and naming to implement this feature,
  UniqueRef/UniqueRefCounted is dropped in favor of Ownable/Owned and
  OwnableRefCounted is used to provide the functions to convert
  between Owned and ARef.
- Link to v4: https://lore.kernel.org/r/20250305-unique-ref-v4-1-a8fdef7b1c2c@pm.me

Changes in v4:
- Just a minor change in naming by request from Andreas Hindborg,
  try_shared_to_unique() -> try_from_shared(),
  unique_to_shared() -> into_shared(),
  which is more in line with standard Rust naming conventions.
- Link to v3: https://lore.kernel.org/r/Z8Wuud2UQX6Yukyr@mango

--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
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  "series": {
    "revision": 8,
    "change-id": "20250305-unique-ref-29fcd675f9e9",
    "prefixes": [],
    "history": {
      "v4": [
        "Z8Wuud2UQX6Yukyr@mango",
        "20250305-unique-ref-v4-1-a8fdef7b1c2c@pm.me"
      ],
      "v5": [
        "20250307-unique-ref-v5-0-bffeb633277e@pm.me"
      ],
      "v6": [
        "20250310-unique-ref-v6-0-1ff53558617e@pm.me"
      ],
      "v7": [
        "20250310-unique-ref-v7-0-4caddb78aa05@pm.me"
      ]
    }
  }
}

Author: Oliver Mangold