Refactor support modules.

tests/test_model.py

yangson/context.py

yangson/datamodel.py

yangson/datatype.py

yangson/enumerations.py

yangson/exceptions.py

yangson/instance.py

yangson/parser.py

yangson/schema.py

yangson/schpattern.py

yangson/statement.py

yangson/typealiases.py

yangson/xpathast.py

yangson/xpathparser.py