Use simple multiplication for squaring. Fixes BUKKIT-4836

This change adds a method to NumberConversions for squaring and
replaces uses of Math.pow(..., 2) with the new method for efficiency
reasons.

By: Wesley Wolfe <weswolf@aol.com>

paper-api/src/main/java/org/bukkit/util/NumberConversions.java

@@ -20,6 +20,10 @@ public final class NumberConversions {
         return floor(num + 0.5d);
     }
 
+    public static double square(double num) {
+        return num * num;
+    }
+
     public static int toInt(Object object) {
         if (object instanceof Number) {
             return ((Number) object).intValue();