ConfCodec: implement as a pair, a-la ConfCodecExT

metaconfig-core/shared/src/main/scala/metaconfig/ConfCodec.scala

@@ -1,21 +1,29 @@
 package metaconfig
 
 trait ConfCodec[A] extends ConfDecoder[A] with ConfEncoder[A] { self =>
-  def bimap[B](in: B => A, out: A => B): ConfCodec[B] = new ConfCodec[B] {
-    override def write(value: B): Conf = self.write(in(value))
-    override def read(conf: Conf): Configured[B] = self.read(conf).map(out)
-  }
+  def bimap[B](in: B => A, out: A => B): ConfCodec[B] =
+    new ConfCodec.Pair[B](encoder.contramap(in), decoder.map(out))
+
+  private[metaconfig] def decoder: ConfDecoder[A]
+  private[metaconfig] def encoder: ConfEncoder[A]
+
 }
 
 object ConfCodec {
   def apply[A](implicit ev: ConfCodec[A]): ConfCodec[A] = ev
+
+  private[metaconfig] class Pair[A](
+      private[metaconfig] val encoder: ConfEncoder[A],
+      private[metaconfig] val decoder: ConfDecoder[A],
+  ) extends ConfCodec[A] {
+    override def write(value: A): Conf = encoder.write(value)
+    override def read(conf: Conf): Configured[A] = decoder.read(conf)
+  }
+
   implicit def EncoderDecoderToCodec[A](implicit
       encode: ConfEncoder[A],
       decode: ConfDecoder[A],
-  ): ConfCodec[A] = new ConfCodec[A] {
-    override def write(value: A): Conf = encode.write(value)
-    override def read(conf: Conf): Configured[A] = decode.read(conf)
-  }
+  ): ConfCodec[A] = new Pair(encode, decode)
 
   val IntCodec: ConfCodec[Int] = ConfCodec[Int]
   val StringCodec: ConfCodec[String] = ConfCodec[String]