Add new WordParserStub class
Add new AssignmentParserBase abstract class
Rename SimpleCommandParserBase class

The SimpleCommandParserBase class is going to be replaced by a new
implementation. The old implementation is temporarily kept in a
different name.
Add EnumSet class template
Make WordParser concrete class

For future maintainability, WordParser is changed from a type alias to a
concrete class. Now WordParser not only parses word components but also
creates a new Word instance.

The test cases for WordParser are removed because the new WordParser is
too simple to test.

AssignmentParserBase and WordParserStub are adapted to the new word
parser definition.
Rename AssignmentParser

The AssignmentParser class is going to be replaced by a new
implementation. The old implementation is temporarily kept in a
different name.
Add new AssignmentParser class