Restore timer_subscribe and delete hello example

Author: torfmaster

examples/hello.rs

@@ -0,0 +1,29 @@
+#![no_std]
+
+use core::fmt::Write;
+use futures::future;
+use libtock::result::TockResult;
+use libtock::timer::Duration;
+use libtock::Drivers;
+
+#[libtock::main]
+async fn main() -> TockResult<()> {
+    let Drivers {
+        console_driver,
+        mut timer_context,
+        ..
+    } = libtock::retrieve_drivers()?;
+    let mut console = console_driver.create_console();
+    let mut with_callback = timer_context.with_callback(|_, _| {
+        writeln!(
+            console,
+            "This line is printed 2 seconds after the start of the program.",
+        )
+        .unwrap();
+    });
+
+    let mut timer = with_callback.init()?;
+    timer.set_alarm(Duration::from_ms(2000))?;
+
+    future::pending().await
+}