Improve expression handling and add parentheses support

internal/ast/ast.go

@@ -63,8 +63,8 @@ type Value struct {
 }
 
 type Expr struct {
-	Segment  Node
-	Logical  Logical
+	First    Node
+	Operator Operator
 	Rest     []Expr
 	Position Position
 }
@@ -73,17 +73,6 @@ func (e Expr) Pos() Position {
 	return e.Position
 }
 
-type ExprSegment struct {
-	Value    Node
-	Operator Operator
-	Rest     []ExprSegment
-	Position Position
-}
-
-func (es ExprSegment) Pos() Position {
-	return es.Position
-}
-
 type FuncCall struct {
 	Name     Ident
 	Params   []Node