Speed up MD5 hash calculation a bit

On an MS Surface 8, the solution for year 2015, day 4,
is sped up from 10 seconds down to 2 seconds duration.

Author: rhaendel

src/main/kotlin/de/ronny_h/aoc/extensions/Hashing.kt

@@ -3,9 +3,11 @@ package de.ronny_h.aoc.extensions
 import java.math.BigInteger
 import java.security.MessageDigest
 
+private val messageDigest = MessageDigest.getInstance("MD5")
+
 /**
  * Converts string to md5 hash.
  */
-fun String.md5() = BigInteger(1, MessageDigest.getInstance("MD5").digest(toByteArray()))
-    .toString(16)
-    .padStart(32, '0')
+fun String.md5() = messageDigest
+        .digest(toByteArray())
+        .toHexString()