tools: add Cancellable, start a futures submodule

To make a future cancellable, use:
let (future, canceller) =
    crate::tools::futures::cancellable(future);

Proceed with using `future` as usual, `canceller` is
clonable and can cancel the future via the `.cancel()`
method.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/tools.rs

@@ -40,6 +40,7 @@ pub mod acl;
 pub mod xattr;
 pub mod vec;
 pub mod io;
+pub mod futures;
 
 mod process_locker;
 pub use process_locker::*;