+ Refine code structure and move code related with control flow `if` into `ifelse_transformer.py`
+ Merge code of `ast_utils.py`  into `utils.py` 

1. copy.deepcopy in NameVisitor should be changed to copy.copy to make hash or set work
2. read_context should be type of gast.Load()/gast.AugLoad(), not gast.Load/gast.AugLoad

Add basic support for while in translating dygraph to static

1. Analysis the variable liveness in class NameVisitor
2. Replace while key word using while_loop API