Implement a part of Expression. (#104)

ConstantNode, RowIDNode, ScoreNode, and ColumnNode.
Enable a test for a part of Expression. (#104)
Implement UnaryNode of Expression. (#104)
Add a missing implementation of Expression::data_type(). (#104)
Enable a test for ConstantNode of Expression. (#104)
Enable a test for ScoreNode of Expression. (#104)
Enable a test for ColumnNode of Expression. (#104)