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 [Mutex][parking_lot::Mutex] from the [parking_lot] crate.
Create an incomplete [ControlledFuture] and explicitly complete it with the
completer:
rust
let (future, completer) = ControlledFuture::<i32>::new();
completer.complete(5).unwrap();
assert_eq!(block_on(future), Ok(5));
Create an initially complete [ControlledFuture] that can be immediately
resolved:
rust
assert_eq!(block_on(ControlledFuture::new_completed(10)), Ok(10));