adds tracing for expressions

[christophkloeffel]

adds tracing for chapter 11

[florianschanda]

Please link to the constructor of ast.Binary_Expression:

• LRM.Expression
• LRM.Relation
• LRM.Simple_Expression
• LRM.Term
• LRM.Factor

Please also link to the constructor of ast.Unary_Expression:

• LRM.Simple_Expression
• LRM.Relation
• LRM.Factor

Please link to the constructor of ast.Range_Test:

• LRM.Relation

Please also link LRM.Primary to the following classes or constructors:

• ast.Integer_Literal
• ast.Decimal_Literal
• ast.String_Literal
• ast.Boolean_Literal
• ast.Null_Literal