Red-black tree abstraction needed by Rust Binder

This patchset contains the red-black tree abstractions needed by the Rust
implementation of the Binder driver.

Binder driver benefits from O(log n) search/insertion/deletion of
key/value mappings in various places, including `process.rs` and
`range_alloc.rs`.  In `range_alloc.rs`, the ability to store and
search by a generic key type is also useful.

Please see the Rust Binder RFC for usage examples [1]. Note that
the `container_of` macro is currently used only by `rbtree` itself.

Users of "rust: rbtree: add red-black tree implementation backed by the C version"
    [PATCH RFC 03/20] rust_binder: add threading support
    [PATCH RFC 05/20] rust_binder: add nodes and context managers
    [PATCH RFC 06/20] rust_binder: add oneway transactions

Users of "rust: rbtree: add iterator"
    [PATCH RFC 17/20] rust_binder: add oneway spam detection

Users of "rust: rbtree: add mutable iterator"
    [PATCH RFC 06/20] rust_binder: add oneway transactions

Users of "rust: rbtree: add `RBTreeCursor`"
    [PATCH RFC 06/20] rust_binder: add oneway transactions

Users of "rust: rbtree: add RBTree::entry"
    Not used in the original RFC, but introduced after further
    code review.  See: https://r.android.com/2849906

The Rust Binder RFC addresses the upstream deprecation of red-black
tree. Quoted here for convenience:

"This RFC uses the kernel's red-black tree for key/value mappings, but we
are aware that the red-black tree is deprecated. We did this to make the
performance comparison more fair, since C binder also uses rbtree for
this. We intend to replace these with XArrays instead. That said, we
don't think that XArray is a good fit for the range allocator, and we
propose to continue using the red-black tree for the range allocator."

Link: https://lore.kernel.org/rust-for-linux/20231101-rust-binder-v1-0-08ba9197f637@google.com/ [1]
To: Miguel Ojeda <ojeda@kernel.org>
To: Alex Gaynor <alex.gaynor@gmail.com>
To: Wedson Almeida Filho <wedsonaf@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@samsung.com>
To: Alice Ryhl <aliceryhl@google.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Arve Hjønnevåg <arve@android.com>
To: Todd Kjos <tkjos@android.com>
To: Martijn Coenen <maco@android.com>
To: Joel Fernandes <joel@joelfernandes.org>
To: Christian Brauner <christian@brauner.io>
To: Carlos Llamas <cmllamas@google.com>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Rob Landley <rob@landley.net>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Michel Lespinasse <michel@lespinasse.org>
Cc: rust-for-linux@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Matt Gilbride <mattgilbride@google.com>

---
Changes in v6:
- EDITME: describe what is new in this series revision.
- EDITME: use bulletpoints and terse descriptions.
- Link to v5: https://lore.kernel.org/r/20240606-b4-rbtree-v5-0-96fe1a0e97c0@google.com

Changes in v5:
- Used `Box::write` in `RBTreeNodeReservation::into_node`, removing
  unnecessary `unsafe` blocks.
- Updated `RBTreeCursor::remove_current` to return the removed node.
- Link to v4: https://lore.kernel.org/r/20240603-b4-rbtree-v4-0-308e43d6abfc@google.com

Changes in v4:
- rebased onto the tip of rust-for-linux/rust-next (97ab3e8)
- addressed comments from draft PR on GitHub: Rust-for-Linux#1081
- Link to v3: https://lore.kernel.org/r/20240418-b4-rbtree-v3-0-323e134390ce@google.com

Changes in v3:
- Address various feedback re: SAFETY and INVARIANT comments from v2.
- Update variable naming and add detailed comments for the `RBTree::insert` (later moved to
  `RBTree::raw_entry`) implementation.
- Link to v2: https://lore.kernel.org/r/20240219-b4-rbtree-v2-0-0b113aab330d@google.com

Changes in v2:
- Update documentation link to the C header file
- Use `core::convert::Infallible` in try_reserve_node
- Link to v1: https://lore.kernel.org/r/20240205-b4-rbtree-v1-0-995e3eee38c0@google.com

--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
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Author: matthewtgilbride