Use initializer lists to construct container<>, lst.

Use initializer lists to construct container<>, lst.

Add constructor of initializer_list<ex> to container<C<ex>>. In
particular, this means that we can finally declare lst objects as
lst{a,2*b,2}. Convert GiNaC to this syntax throughout. Deprecate
the old constructors taking 1..16 ex parameters and the ones from
comma seaparated lists (without braces).