Performance boost with char array instead of String operations

The private method `consistentShuffle(String,String)` operates with different methods of the `String` parameter `alphabet` like
```java
substring(int, char)
charAt(int)
```
which can be replaced with a `char[]` instead of a `char` as temporary variable.

This removes *five* additional `String` operations/methods and *six* `String` concatenations to *three* simple array assignments.