serializer.tcl

* Hopefully the last big naming change: Instead of writing "C class-object method foo {} {...}" one can write now "C class method foo {} {...}" to define a class method named "foo" for class "C". This naming change became possible by defining XOTcl's "class" (and "superclass") as object parameter only. To change a class of an object o, one has to use "o configure -class NEWCLASS". The term "object-class" looks alien to language beginners, the term "class" is much more straightforward. Changing classes or superclasses is seldomly used by typicall application programs.
For already existing nx scripts, changing "object-class" into
class should be straightforward.

* various documentation updates (migration guide, nx tutorial)