Support SMT-LIB's str.to_code and str.from_code functions over strings

[RyanGlScott]

Currently, what4's symbolic String API does not really offer a way to convert individual characters in a string to bytes (or vice versa). SMT-LIB 2's str.to_code and str.from_code functions offer a way to accomplish this. str.to_code converts a single-character string to its code point (represented as an Integer), and str.from_code converts a code point back to a string. We should offer what4 versions of these functions.