A Future value that resolves once it's explicitly completed, potentially
from a different thread or task, similar to Java's CompletableFuture.
Currently, this is implemented using the Mutex from the std crate.
Create an incomplete Completable and explicitly complete it with the
completer:
rust
let (future, completer) = Completable::<i32>::new();
completer.complete(5).unwrap();
assert_eq!(block_on(future), Ok(5));
Create an initially complete Completable that can be immediately
resolved:
rust
assert_eq!(block_on(Completable::new_completed(10)), Ok(10));