Use Fraction's private attributes in arithmetic methods

Use Fraction's private attributes in arithmetic methods

That's slightly faster for small components.

Before:
$ ./python -m timeit -r11 -s 'from fractions import Fraction as F' \
-s 'a = F(10, 3)' -s 'b = F(6, 5)' 'a + b'
20000 loops, best of 11: 12.1 usec per loop
$ ./python -m timeit -r11 -s 'from fractions import Fraction as F' \
-s 'a = F(10, 3)' -s 'b = 7' 'a + b'
20000 loops, best of 11: 11.4 usec per loop

After:
$ ./python -m timeit -r11 -s 'from fractions import Fraction as F' \
-s 'a = F(10, 3)' -s 'b = F(6, 5)' 'a + b'
50000 loops, best of 11: 9.74 usec per loop
$ ./python -m timeit -r11 -s 'from fractions import Fraction as F' \
-s 'a = F(10, 3)' -s 'b = 7' 'a + b'
20000 loops, best of 11: 16.5 usec per loop

On the master:
$ ./python -m timeit -r11 -s 'from fractions import Fraction as F' \
-s 'a = F(10, 3)' -s 'b = F(6, 5)' 'a + b'
50000 loops, best of 11: 9.61 usec per loop
$ ./python -m timeit -r11 -s 'from fractions import Fraction as F' \
-s 'a = F(10, 3)' -s 'b = 7' 'a + b'
50000 loops, best of 11: 9.11 usec per loop