Implemented function that replaces "\?" into its '\?' counterpart

Specifically, a string littered with actual backslashes and
characters is modified so that actual escape codes are replaced
with their meaning ("\n" -> '\n'), and non-escape code escaped
characters are replaced with themselves ("\j" -> 'j'). A unittest
which possibly non-exhaustively but probably exhaustively tests the
function was also implemented.
Single/Double quote operators now recognize escape codes in strings