Add forwarding Word constructor
Rename WordParserStub class

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