add div axioms

[Daejun Park]

for each uninterpreted abstraction t = f_div(x,y):

add the following axioms:

t == 0  if y == 0

t == 0  if x < y
t == 1  if x == y > 0

t * y <= x
t * y > x - y  if x >= y > 0

no overflow in bitvector arithmetic.

need to evaluate different ways of instantiating conditional (in)equalities.