Fix nitpicks in Mining plugin

Also adds equals() and hashcode() implementations to the various
Mining actions.

game/plugin/skills/mining/src/gem.kt

@@ -1,12 +1,13 @@
 package org.apollo.game.plugin.skills.mining
 
-enum class Gem(val id: Int, val chance: Int) {
-    UNCUT_SAPPHIRE(1623, 0),
-    UNCUT_EMERALD(1605, 0),
-    UNCUT_RUBY(1619, 0),
-    UNCUT_DIAMOND(1617, 0)
-}
+enum class Gem(val id: Int) { // TODO add gem drop chances
+    UNCUT_SAPPHIRE(1623),
+    UNCUT_EMERALD(1605),
+    UNCUT_RUBY(1619),
+    UNCUT_DIAMOND(1617);
 
-val GEMS = Gem.values()
-
-fun lookupGem(id: Int): Gem? = GEMS.find { it.id == id }
+    companion object {
+        private val GEMS = Gem.values().associateBy({ it.id }, { it })
+        fun lookup(id: Int): Gem? = GEMS[id]
+    }
+}