Move a bunch of things around and flatten common crate

backend/common/src/time.rs

@@ -0,0 +1,9 @@
+/// Returns current unix time in ms (compatible with JS Date.now())
+pub fn now() -> u64 {
+    use std::time::SystemTime;
+
+    SystemTime::now()
+        .duration_since(SystemTime::UNIX_EPOCH)
+        .expect("System time must be configured to be post Unix Epoch start; qed")
+        .as_millis() as u64
+}