[cfg] support process variable declarations
Some HDL (like VHDL) support variables declaration not only at the module level, but also on process level.
For the CFG represenation to be universal enough, the support for adding variable declarations to not-only-module CFG nodes should be added.
For this task an abstract class like CfgVariableContainerNode shpuld be created. Other nodes like Module/Process should extend it.