parser: use standard functions to parse strings

modified the get string function to use standard library functions to
parse string constants; the input string is temporarily put into a
standard input string stream and read form it, and as characters of the
string are parsed, they are moved from the input stream into a local
standard string that is passed (moved) to the token constructor

modified the token constructor for creating a string constant token to
take a standard string, though for now it is converted to a c-style
string to initialize the QString member variable