parser: use standard functions to parse numbers

modified the get number function to use standard library functions to
parse number and convert them; the input string is temporarily put into
a standard input string stream and read from it (the entire parser will
be changed to this) and as characters of the number are parsed, they are
moved from the input stream into a local standard string that will be
passed (moved) to the token constructors when the token is created

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