`eval_str'
----------

eval_str(STR)
     :: Evaluates a string STR.

RETURN
     object

STR
     string which can be accepted by Asir parser

   * This function evaluates a string which can be accepted by Asir
     parser and returns the result.

   * The input string should represent an expression.

   * This functions is the inversion function of `rtostr()'.

     [0] eval_str("1+2");
     3
     [1] fctr(eval_str(rtostr((x+y)^10)));
     [[1,1],[x+y,10]]

References
     *Note `rtostr': rtostr

