Make .eval() evaluate top-level only.

Make .eval() evaluate top-level only.

With the previous patch, some old workarounds have become unnecessary:
If all expressions are evaluated, any object which is an aggregate of
expressions will only ever have to evaluate the top level. As such, it
has become pointless to evaluate child objects of an expression prior
to doing its own term-rewriting. This patch removes the evaluation of
children from all GiNaC objects. It also removes the now superfluous
parameter 'level' of the eval methods.