Function fix_double 

pub fn fix_double(str: &str, fill: char) -> String

Replaces consecutive identical characters by inserting a fill character between them.

For example, “AA” becomes “AQA” if the fill character is ‘Q’. This is often used in classical ciphers to handle double letters.

NOTE: this function works regardless of alignment; that is, it does not check if the double letters are on a 2-character boundary. Therefore, it is useless for Playfair, or any bi-grammatic ciphers.

e.g. HELLOWORLD -> HE LX LO WO RL DX but also CETOOTEST -> CE TO XO TE ST

Arguments

• str - The input string to process.
• fill - The character to insert between identical consecutive characters.

Returns

A new String with the fill character inserted where necessary.