singleton: Singleton decorator

Add a decorator to implement the singleton design pattern.
event: C# like event handling

Add C# like event handling.
timeutil: Time utilities

Add a function to return monotonic time in seconds.
Return exitcode as well as status if asked

run_sync() now returns exitcode as well as status boolean if the
return_exitcode argument is True.
builtins: Provide an ArgumentParser with builtin name and description

Display the builtin name in the usage help.