extract a couple of collision functions to a separate module for reuse

fcore/collision.sml

@@ -0,0 +1,15 @@
+structure Collision =
+struct
+  fun isColliding (ix, iy, ifx, ify, cx, cy, cfx, cfy) =
+    ix < cfx andalso ifx > cx andalso iy < cfy andalso ify > cy
+
+  fun isCollidingPlus (ix, iy, iw, ih, cx, cy, cw, ch) =
+    let
+      val ifx = ix + iw
+      val ify = iy + ih
+      val cfx = cx + cw
+      val cfy = cy + ch
+    in
+      isColliding (ix, iy, ifx, ify, cx, cy, cfx, cfy)
+    end
+end