Redone the way in which function serial numbers are stored. This allows for

Redone the way in which function serial numbers are stored. This allows for
using namespace qualifiers in the is_ex_the_function() macro like this

  is_ex_the_function(e, GiNaC::sin);

and it also works with the (overloaded) zeta() and psi() functions.
Unfortunately, it's not possible to

  using GiNaC::sin;
  is_ex_the_function(e, sin);

and the is_the_function<> template is not useful outside the is_ex_the_function()
macro. Even renaming the 'sin_SERIAL' class to 'sin' (which doesn't seem to
work with gcc3) you would still have to write 'is_the_function<class sin>(e)'
instead of 'is_the_function<sin>(e)'. I currently see no better solution,
though...