change arrow keys (left, down, right) to letter keys (s, d, f) and arrow up to letter key 'k', because I think it is more comfortable

shell/gl-draw.sml

@@ -173,7 +173,7 @@ struct
           val _ = draw shellState
 
           val _ = Glfw.swapBuffers window
-          val _ = Glfw.waitEvents ()
+          val _ = Glfw.pollEvents ()
         in
           helpLoop (shellState, game)
         end

shell/input-state.sml

@@ -29,19 +29,19 @@ struct
   open Input
 
   fun handleKey (key, action) =
-    if key = ARROW_UP then
+    if key = KEY_K then
       if action = PRESS then (#upHeld state) := true
       else if action = RELEASE then (#upHeld state) := false
       else ()
-    else if key = ARROW_DOWN then
+    else if key = KEY_D then
       if action = PRESS then (#downHeld state) := true
       else if action = RELEASE then (#downHeld state) := false
       else ()
-    else if key = ARROW_LEFT then
+    else if key = KEY_S then
       if action = PRESS then (#leftHeld state) := true
       else if action = RELEASE then (#leftHeld state) := false
       else ()
-    else if key = ARROW_RIGHT then
+    else if key = KEY_F then
       if action = PRESS then (#rightHeld state) := true
       else if action = RELEASE then (#rightHeld state) := false
       else ()