Initial approach for unifying Flag and Register

Related to #3 (closed)

As seen, much code can be removed by inheritance. Probably more.

src/simudator/core/modules/__init__.py

 from .demux import Demux
-from .flag import Flag
 from .memory import Memory
 from .mux import Mux
-from .register import Register
+from .register import Flag, IntegerRegister, Register
 
-__all__ = ["Demux", "Flag", "Memory", "Mux", "Register"]
+__all__ = ["Demux", "Flag", "IntegerRegister", "Memory", "Mux", "Register"]