[with patch, positive review] ivalue field in integer_mod.pyx shouldn't be public

[craigcitro]

The ivalue field for IntegerMod_int is public, but it shouldn't be. The following is very frightening, for instance:

sage: R = Integers(10)
sage: x = R(2)
sage: x
2
sage: x.ivalue = 33
sage: x
33
sage: R(2)
33

It's easy to make this field no longer be public, but lots of things are using the fact that it is, so one needs to go through and make everything work correctly again.

[craigcitro]

The attached patch fixes this issue, and in fact, gives about a 1.5-2X speedup on multiplying IntegerMod_ints. The interesting part is that previous to this patch, the ivalue field was occasionally being accessed as a Python attribute instead of a Cython attribute! That's why it broke if we made the field not public in the first place. Oops.

BEFORE:

sage: R = Integers(100) ; x = R(3) ; y = R(5)
sage: timeit('x*y')
625 loops, best of 3: 403 ns per loop
sage: timeit('x*y')
625 loops, best of 3: 370 ns per loop
sage: timeit('x*y')
625 loops, best of 3: 410 ns per loop
sage: timeit('x*y')
625 loops, best of 3: 405 ns per loop

AFTER:

sage: R = Integers(100) ; x = R(3) ; y = R(5)
sage: timeit('x*y')
625 loops, best of 3: 190 ns per loop
sage: timeit('x*y')
625 loops, best of 3: 213 ns per loop
sage: timeit('x*y')
625 loops, best of 3: 174 ns per loop
sage: timeit('x*y')
625 loops, best of 3: 191 ns per loop

[mabshoff]

Merged in Sage 3.3.alpha1

Cheers,

Michael