add data method to frozen::string
add data method to frozen::string

This is to give a way to access the data_ member, which
might be needed by users of the class. They could already
do `&str[0]` anyways, this is just a more pleasant syntax.

We don't call it `c_str` because the class doesn't enforce
that the string is null-terminated.

serge-sans-paille/frozen,serge-sans-paille/frozen