Move rtic macros to repo root, tune xtask

rtic-macros/ui/task-init.rs

@@ -0,0 +1,17 @@
+#![no_main]
+
+#[rtic_macros::mock_app(device = mock)]
+mod app {
+    #[shared]
+    struct Shared {}
+
+    #[local]
+    struct Local {}
+
+    #[init]
+    fn foo(_: foo::Context) -> (Shared, Local) {}
+
+    // name collides with `#[idle]` function
+    #[task]
+    async fn foo(_: foo::Context) {}
+}