Refactor ruin_app, add some README files, minor usability in reactivity

lib/runtime/src/platform/linux_x86_64/runtime.rs

@@ -54,6 +54,12 @@ pub struct TimeoutHandle {
     _local: Rc<()>,
 }
 
+impl TimeoutHandle {
+    pub fn clear(&self) {
+        clear_timeout(self);
+    }
+}
+
 #[derive(Clone)]
 /// Handle returned by [`set_interval`].
 pub struct IntervalHandle {
@@ -62,6 +68,12 @@ pub struct IntervalHandle {
     _local: Rc<()>,
 }
 
+impl IntervalHandle {
+    pub fn clear(&self) {
+        clear_interval(self);
+    }
+}
+
 /// Handle returned by [`queue_future`].
 ///
 /// Awaiting a join handle yields the output of the queued future.
@@ -72,9 +84,10 @@ pub struct JoinHandle<T> {
 /// Future returned by [`yield_now`].
 ///
 /// Awaiting this future will immediately yield control back to the runtime scheduler, allowing other queued microtasks
-/// to run before the current task continues executing. Note that continuation of futures runs as a microtask, so this
+/// to run before the current task continues executing. Note that continuations of futures run as microtasks, so this
 /// can only yield to other microtasks and not to macrotasks (driver events such as file or network I/O, timers, or
-/// channel messages).
+/// channel messages). To yield to macrotasks, you must allow the flow of execution to return to the runtime event loop
+/// and flush the full microtask queue, for example by awaiting a timer.
 pub struct YieldNow {
     yielded: bool,
 }