add a unix_timestamp extern (tgstation#122)

Co-authored-by: Mothblocks <[email protected]>

Author: ZephyrTFA

dmsrc/time.dm

@@ -1,3 +1,6 @@
 #define rustg_time_microseconds(id) text2num(RUSTG_CALL(RUST_G, "time_microseconds")(id))
 #define rustg_time_milliseconds(id) text2num(RUSTG_CALL(RUST_G, "time_milliseconds")(id))
 #define rustg_time_reset(id) RUSTG_CALL(RUST_G, "time_reset")(id)
+
+/proc/rustg_unix_timestamp()
+	return text2num(RUSTG_CALL(RUST_G, "unix_timestamp")())

src/time.rs

@@ -35,3 +35,15 @@ byond_fn!(fn time_reset(instant_id) {
         Some("")
     })
 });
+
+byond_fn!(
+    fn unix_timestamp() {
+        Some(
+            std::time::SystemTime::now()
+                .duration_since(std::time::UNIX_EPOCH)
+                .unwrap()
+                .as_secs_f64()
+                .to_string(),
+        )
+    }
+);