Fix uninitialized variables bug

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Refine toplevel API

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Define implies using ite operator. Rename mk_bin_op to mk_bin_rop (it is using right associativity). Add mk_bin_lop (for left assoc).

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>