rust: add dynamic ID pool abstraction for bitmap

This is a port of the Binder data structure introduced in commit
15d9da3f81 ("binder: use bitmap for faster descriptor lookup") to
Rust.

Like drivers/android/dbitmap.h, the ID pool abstraction lets
clients acquire and release IDs. The implementation uses a bitmap to
know what IDs are in use, and gives clients fine-grained control over
the time of allocation. This fine-grained control is needed in the
Android Binder. We provide an example that release a spinlock for
allocation and unit tests (rustdoc examples).

The implementation does not permit shrinking below capacity below
BITS_PER_LONG.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Suggested-by: Yury Norov <yury.norov@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Burak Emir <bqe@google.com>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>

rust/kernel/lib.rs

@@ -90,6 +90,7 @@ pub mod faux;
 pub mod firmware;
 pub mod fmt;
 pub mod fs;
+pub mod id_pool;
 pub mod init;
 pub mod io;
 pub mod ioctl;