rust: add an intrusive linked list

It is unsafe because it requires users to track if/when objects are
inserted into or removed from lists.

It is meant to be used as the basis for other linked lists, so the
`linked_list` module will be updated to use it, `raw_list` will be
removed, and a non-intrusive list of owned entries will be written based
on it.

It's also used in the workqueue-based executor to keep track of all
tasks -- it is helpful here because it is cheaper than the other
options.

It comes with a set of tests that run with `make rusttest`.

Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>

Author: wedsonaf

rust/kernel/lib.rs

@@ -68,6 +68,7 @@ pub mod task;
 pub mod linked_list;
 mod raw_list;
 pub mod rbtree;
+pub mod unsafe_list;
 
 #[doc(hidden)]
 pub mod module_param;