Add DFGOperator.evaluate method

This adds the evaluate method to operators. Operators are objects
that encapsulate the mathematical operation which appear as a result
of parsing.

During the transformation stage, these operations are evaluated
calling the `evaluate` method.

spec/dfgoperator_spec.cr

@@ -0,0 +1,10 @@
+require "spec"
+require "../src/dfg.cr"
+require "../src/dfgoperator.cr"
+
+describe Isekai do
+    op : DFGOperator = OperatorAdd.new()
+    op.evaluate(1, 2).should eq 3
+    op = OperatorMul.new()
+    op.evaluate(1, 2).should eq 2
+end

src/dfgoperator.cr

@@ -1,40 +1,76 @@
 # Operators on DFGExpressions. Wraps the operation in a type-safe manner.
-class DFGOperator
+abstract class DFGOperator
+    abstract def evaluate (left : T, right : T) forall T
 end
 
 class LeftShiftOp < DFGOperator
     def initialize (@bitwidth : Int32)
     end
+
+    def evaluate (left : T, right : T) forall T
+        return left << right
+    end
 end
 
 class RightShiftOp < DFGOperator
     def initialize (@bitwidth : Int32)
     end
+
+    def evaluate (left : T, right : T) forall T
+        return left >> right
+    end
 end
 
 class LogicalAndOp < DFGOperator
+    def evaluate (left : T, right : T) forall T
+        return left && right
+    end
 end
 
 class OperatorAdd < DFGOperator
+    def evaluate (left : T, right : T) forall T
+        return left + right
+    end
 end
 
 class OperatorSub < DFGOperator
+    def evaluate (left : T, right : T) forall T
+        return left - right
+    end
 end
 
 class OperatorMul < DFGOperator
+    def evaluate (left : T, right : T) forall T
+        return left * right
+    end
 end
 
 class OperatorDiv < DFGOperator
+    def evaluate (left : T, right : T) forall T
+        return left / right
+    end
 end
 
 class OperatorMod < DFGOperator
+    def evaluate (left : T, right : T) forall T
+        return left % right
+    end
 end
 
 class OperatorXor < DFGOperator
+    def evaluate (left : T, right : T) forall T
+        return left ^ right
+    end
 end
 
 class OperatorBor < DFGOperator
+    def evaluate (left : T, right : T) forall T
+        return left | right
+    end
 end
 
 class OperatorBAnd < DFGOperator
-end
+    def evaluate (left : T, right : T) forall T
+        return left & right
+    end
+end