add closure example that is passed to the host

examples/closure/platform/src/lib.rs

@@ -0,0 +1,38 @@
+use std::mem::MaybeUninit;
+use std::time::SystemTime;
+
+extern "C" {
+    #[link_name = "closure_1_exposed"]
+    fn closure(output: *mut u8) -> ();
+
+    #[link_name = "closure_1_size"]
+    fn closure_size() -> i64;
+}
+
+#[no_mangle]
+pub fn rust_main() -> isize {
+    println!("Running Roc closure");
+    let start_time = SystemTime::now();
+    let (function_pointer, closure_data) = unsafe {
+        let mut output: MaybeUninit<(fn(i64) -> i64, i64)> = MaybeUninit::uninit();
+
+        closure(output.as_mut_ptr() as _);
+
+        output.assume_init()
+    };
+    let answer = function_pointer(closure_data);
+    let end_time = SystemTime::now();
+    let duration = end_time.duration_since(start_time).unwrap();
+
+    println!(
+        "Roc closure took {:.4} ms to compute this answer: {:?}",
+        duration.as_secs_f64() * 1000.0,
+        // truncate the answer, so stdout is not swamped
+        answer
+    );
+
+    println!("closure size {:?}", unsafe { closure_size() });
+
+    // Exit code
+    0
+}