Make Thing extend ThingPicker.

By turning things into their own thing pickers, we can avoid creating a
bunch of SingleThingPicker wrappers, which are now redundant. Because
the semantics of using things and thing pickers are quite different, it
does perhaps make it necessary to be a bit more careful about picking
the right type.

src/main/java/com/garbagemule/MobArena/things/SingleThingPicker.java

@@ -1,21 +0,0 @@
-package com.garbagemule.MobArena.things;
-
-public class SingleThingPicker implements ThingPicker {
-
-    private final Thing thing;
-
-    public SingleThingPicker(Thing thing) {
-        this.thing = thing;
-    }
-
-    @Override
-    public Thing pick() {
-        return thing;
-    }
-
-    @Override
-    public String toString() {
-        return thing.toString();
-    }
-
-}

src/main/java/com/garbagemule/MobArena/things/Thing.java

@@ -12,8 +12,12 @@ import org.bukkit.entity.Player;
  * The interface exposes three methods that are all optional operations. An
  * operation returns false if it fails or if it isn't applicable to the given
  * thing (which is the same as failing).
+ * <p>
+ * A thing is automatically a {@link ThingPicker} for itself, which means it
+ * can be used in any place that a thing picker is expected, avoiding the need
+ * to wrap things in a "dummy picker".
  */
-public interface Thing {
+public interface Thing extends ThingPicker {
 
     /**
      * Give this thing to the given player.
@@ -39,4 +43,9 @@ public interface Thing {
      */
     boolean heldBy(Player player);
 
+    @Override
+    default Thing pick() {
+        return this;
+    }
+
 }

src/main/java/com/garbagemule/MobArena/things/ThingPickerManager.java

@@ -25,8 +25,7 @@ public class ThingPickerManager implements ThingPickerParser {
                 return picker;
             }
         }
-        Thing thing = things.parse(s);
-        return new SingleThingPicker(thing);
+        return things.parse(s);
     }
 
 }