upgrade Hash in the same style as done in typelevel/cats#1997

In this commit, the "on" method was removed from cat's Eq trait.
The "by" method which was using that method was reimplemented..

src/main/scala/com/rklaehn/radixtree/Hash.scala

@@ -15,15 +15,6 @@ import scala.util.hashing.Hashing
   */
 trait Hash[@sp A] extends Any with Eq[A] with Serializable { self =>
   def hash(a: A): Int
-  /**
-    * Constructs a new `Eq` instance for type `B` where 2 elements are
-    * equivalent iff `eqv(f(x), f(y))`.
-    */
-  def on[@sp B](f: B => A): Hash[B] =
-    new Hash[B] {
-      def eqv(x: B, y: B): Boolean = self.eqv(f(x), f(y))
-      def hash(x: B): Int = self.hash(f(x))
-    }
 }
 
 trait HashFunctions {
@@ -83,7 +74,10 @@ object Hash extends HashFunctions {
     * using the given function `f`.
     */
   def by[@sp A, @sp B](f: A => B)(implicit ev: Hash[B]): Hash[A] =
-    ev.on(f)
+    new Hash[A] {
+      def eqv(x: A, y: A): Boolean = ev.eqv(f(x), f(y))
+      def hash(x: A): Int = ev.hash(f(x))
+    }
 
   /*
   /**