public String encrypt$($String input$)$

Applies this Substitution Cipher to the input, returning the result.

Applying the Substitution Cipher is defined as replacing each input character with the corresponding character in shifter at the same relative position within the ordered encodable range.

In general, you may not assume anything about the input other than all characters being in the encodable range.

Since we're allowing clients to set a shifter after construction, this method should throw an IllegalStateException if the shifter is null or empty.