Make sure add::eval() collects all numeric terms.

Make sure add::eval() collects all numeric terms.

Apparently, add::eval() assumed that none of the elements of its epvector
has a numeric rest. However, nothing guarantees that -- in particular
evalchildren() doesn't (and actually cannot) do so. Since there are many
places where a new add is constructed directly from an epvector, enforcing
this doesn't make sense either. One example where it did fail was found by
Burgin Erocal: real_part(1+2*(sqrt(2)+1)*(sqrt(2)-1)) returned 1+2*1, not 3.

Thanks to Burcin Erocal for reporting this bug.