Move config binary and GHC intermediate outputs to cacheDir

That's where they belong. As XDG was more or less broken in previous
xmonad releases, we can assume few people use it so now's the best time
to move files around.

For users of `~/.xmonad`, this only causes intermediate outputs (.o,
.hi) to go elsewhere.

Fixes: https://github.com/xmonad/xmonad/issues/178

CHANGES.md

@@ -29,6 +29,10 @@
     In the cases of 1. and 3., the build script or executable is
     expected to be in the config dir.
 
+    Additionally, the xmonad config binary and intermediate object files were
+    moved to the cache directory (only relevant if using XDG or
+    `XMONAD_CACHE_DIR`).
+
   * Change `ScreenDetail` to a newtype and make `RationalRect` strict in
     its contents.