Annotate Command and Subscription with #[must_use]

Calling a function returning one of these types without using it is
almost certainly a mistake. Luckily Rust's `#[must_use]` can help warn
about this.

futures/src/command.rs

@@ -1,4 +1,5 @@
 /// A set of asynchronous actions to be performed by some runtime.
+#[must_use = "`Command` must be returned to runtime to take effect"]
 #[derive(Debug)]
 pub struct Command<T>(Internal<T>);