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

lib/reactivity/examples/readme_md_01.rs

@@ -0,0 +1,31 @@
+use std::time::Duration;
+
+use ruin_reactivity::{cell, effect};
+use ruin_runtime::{main, set_timeout};
+
+#[main]
+fn main() {
+    // Creates an observable value. Calling `.get` from within an observer will create a dependency, and calling `.set`
+    // will trigger updates to any dependent observers.
+    let v = cell(5);
+
+    // Creates an observer that prints the value of `v` whenever it changes.
+    // Calling `.leak()` on the effect handle allows it to run for the lifetime of the program without automatically
+    // disposing when dropped.
+    effect({
+        let v = v.clone();
+        move || {
+            println!("v is: {}", v.get());
+        }
+    })
+    .leak();
+
+    // Queue a future to wait 5 seconds and then update `v`. This will trigger
+    // the effect to run again and print the new value.
+    set_timeout(Duration::from_secs(5), {
+        let v = v.clone();
+        move || {
+            v.set(v.get() + 20);
+        }
+    });
+}