rust: types: add Either type

Introduce the new `types` module of the `kernel` crate with
`Either` as its first type.

`Either<L, R>` is a sum type that always holds either a value
of type `L` (`Left` variant) or `R` (`Right` variant).

For instance:

    struct Executor {
        queue: Either<BoxedQueue, &'static Queue>,
    }

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/lib.rs

@@ -31,6 +31,7 @@ mod static_assert;
 #[doc(hidden)]
 pub mod std_vendor;
 pub mod str;
+pub mod types;
 
 #[doc(hidden)]
 pub use bindings;