Simplified constant folding

src/expression/Expression.go

@@ -11,9 +11,8 @@ type Expression struct {
 	Parent     *Expression
 	Children   []*Expression
 	Token      token.Token
-	Value      int
 	Precedence int8
-	IsFolded   bool
+	Foldable
 }
 
 // New creates a new expression.
@@ -62,8 +61,8 @@ func (expr *Expression) Reset() {
 	}
 
 	expr.Token.Reset()
-	expr.Value = 0
 	expr.Precedence = 0
+	expr.Value = 0
 	expr.IsFolded = false
 }