Implemented an abstract syntax tree

src/build/expression/bench_test.go

@@ -12,7 +12,6 @@ func BenchmarkExpression(b *testing.B) {
 	tokens := token.Tokenize(src)
 
 	for i := 0; i < b.N; i++ {
-		expr := expression.Parse(tokens)
-		expr.Close()
+		expression.Parse(tokens)
 	}
 }