better support domains and program blocks

[maeder]

programs are only legal over Cpos, that may be introduced by free domains

* add keyword "domain" and create Cpo data types

* better extract variable declarations from the lhs of a program equation, so that (var s : s) is not necessary in https://svn-agbkb.informatik.uni-bremen.de/viewcvs/Hets-lib/trunk/HasCASL/Metatheory/Interpreter.het (this may solve parts of #433, too)