Make add::eval(), mul::eval() work without compromise.

Make add::eval(), mul::eval() work without compromise.

If a user asks to evaluate an object, the expectation is that the
library returns an evaluated, canonical expression. Everything else is
a bug. (It doesn't matter whether the user asks explicitly or by
assigning to an ex.) It turns out that it was possible to construct
objects which didn't fully evaluate. This patch fixes this by making
eval() a little bit more careful.

This obsoletes the need to go through combine_ex_with_coeff_to_pair()
when constructing an epvector that is then used to construct an add
or mul. (Alas, this was omitted frequently enough...)