return a number parsed from the contents of string, or a part of it. from and to specify the part of the string; to = nil means the end of it. radix defaults to decimal. if the string or part doesn't start with a number, nil is returned. the second value is the index in string of the first non-digit, or nil if none. fail-if-not-whole-string means return nil and 0 unless the whole string or specified part can be parsed as a number.