[9_4] use union instead of strange static_cast

[da-liii]

改成union有什么具体的好处么？除了纯粹的代码重构？

[jingkaimori]

@darcy-shen 给url的构造提升了一点性能：

Before

ns/op	op/s	err%	total	benchmark
5,910.40	169,193.15	4.7%	0.01	url construct 8
4,576.62	218,501.70	0.2%	0.01	url descends equal
6,551.03	152,647.65	3.7%	0.01	url descends concat 2
16,606.90	60,215.95	1.6%	0.01	url descends concat 8

After

ns/op	op/s	err%	total	benchmark
5,685.55	175,884.51	0.8%	0.01	url construct 8
4,501.29	222,158.66	4.6%	0.01	url descends equal
6,548.65	152,703.26	4.6%	0.01	url descends concat 2
16,521.43	60,527.45	4.3%	0.01	url descends concat 8

别的提升体现不出来，因为很多功能的测试都没有涵盖到STRING标签的树节点能否有子节点

[jingkaimori]

Before

complexityN	ns/op	op/s	err%	total	benchmark
1	87.65	11,409,176.54	0.2%	0.21	construct atomic tree
1	485.33	2,060,441.77	0.6%	1.17	construct compound tree of depth from argument
1	510.27	1,959,754.37	0.6%	1.23	construct compound tree from array
1	758.74	1,317,974.01	0.6%	1.82	construct compound tree of given depth
2	1,132.45	883,044.27	0.3%	2.71	construct compound tree of given depth
3	1,440.51	694,198.03	0.5%	3.44	construct compound tree of given depth
4	1,741.73	574,143.44	0.4%	4.15	construct compound tree of given depth
5	2,044.63	489,085.42	0.8%	4.88	construct compound tree of given depth
1	7.06	141,596,076.39	0.4%	0.02	type of tree
2	7.13	140,168,861.65	2.0%	0.02	type of tree
3	7.10	140,800,335.90	0.3%	0.02	type of tree
4	7.24	138,193,748.81	1.6%	0.02	type of tree
5	6.73	148,611,899.85	3.3%	0.02	type of tree
1	11.00	90,947,829.86	2.0%	0.03	size of tree
2	11.20	89,266,083.46	1.9%	0.03	size of tree
3	10.98	91,066,513.76	1.7%	0.03	size of tree
4	11.13	89,863,538.92	4.0%	0.03	size of tree
5	10.84	92,278,175.35	2.6%	0.03	size of tree
1	9.35	106,966,018.21	3.0%	0.02	immutable index of tree
2	9.56	104,551,842.95	2.1%	0.02	immutable index of tree
3	9.46	105,681,178.15	2.4%	0.03	immutable index of tree
4	9.59	104,308,413.12	1.8%	0.02	immutable index of tree
5	9.46	105,661,628.22	0.6%	0.02	immutable index of tree
1	126.08	7,931,695.54	0.2%	0.30	mutable index of tree
2	126.42	7,909,896.98	0.2%	0.30	mutable index of tree
3	126.47	7,906,751.03	0.4%	0.30	mutable index of tree
4	125.88	7,944,076.77	0.1%	0.30	mutable index of tree
5	127.65	7,833,959.51	0.9%	0.31	mutable index of tree
1	1,024.10	976,464.64	0.5%	2.45	copy tree
2	1,020.08	980,315.75	0.4%	2.44	copy tree
3	1,020.08	980,314.64	1.0%	2.41	copy tree
4	1,021.74	978,726.77	1.2%	2.41	copy tree
5	1,029.78	971,076.98	0.2%	2.46	copy tree
1	408.35	2,448,875.16	0.4%	0.98	equality of tree
2	410.07	2,438,614.30	0.3%	0.98	equality of tree
3	409.97	2,439,202.39	0.3%	0.98	equality of tree
4	411.24	2,431,650.77	0.5%	0.99	equality of tree
5	411.54	2,429,912.22	0.6%	0.99	equality of tree

After

complexityN	ns/op	op/s	err%	total	benchmark
1	83.63	11,956,977.75	0.2%	0.20	construct atomic tree
1	471.90	2,119,114.03	0.5%	1.13	construct compound tree of depth from argument
1	473.76	2,110,778.14	0.2%	1.13	construct compound tree from array
1	700.86	1,426,826.87	0.4%	1.68	construct compound tree of given depth
2	1,057.22	945,872.47	0.2%	2.53	construct compound tree of given depth
3	1,426.49	701,022.54	0.2%	3.41	construct compound tree of given depth
4	1,695.10	589,935.52	0.4%	4.06	construct compound tree of given depth
5	1,936.56	516,378.66	0.1%	4.62	construct compound tree of given depth
1	6.40	156,141,445.66	1.2%	0.02	type of tree
2	6.56	152,364,108.27	2.4%	0.02	type of tree
3	6.56	152,420,623.15	3.5%	0.02	type of tree
4	6.56	152,408,961.73	3.3%	0.02	type of tree
5	6.72	148,797,187.52	2.7%	0.02	type of tree
1	10.45	95,692,393.95	2.9%	0.03	size of tree
2	10.47	95,489,113.77	2.3%	0.03	size of tree
3	10.59	94,421,383.65	0.7%	0.03	size of tree
4	10.58	94,535,216.47	0.5%	0.03	size of tree
5	10.53	94,952,031.95	0.1%	0.03	size of tree
1	9.18	108,900,726.14	0.2%	0.02	immutable index of tree
2	9.20	108,710,582.01	0.3%	0.02	immutable index of tree
3	9.23	108,307,334.36	0.5%	0.02	immutable index of tree
4	9.19	108,788,564.96	0.4%	0.02	immutable index of tree
5	9.20	108,720,063.44	0.1%	0.02	immutable index of tree
1	124.70	8,019,336.57	2.2%	0.29	mutable index of tree
2	120.40	8,305,652.04	0.9%	0.29	mutable index of tree
3	119.31	8,381,302.47	0.8%	0.29	mutable index of tree
4	118.39	8,446,632.92	1.0%	0.29	mutable index of tree
5	118.64	8,429,087.70	0.3%	0.29	mutable index of tree
1	893.11	1,119,678.75	0.2%	2.14	copy tree
2	903.81	1,106,427.67	0.2%	2.16	copy tree
3	893.81	1,118,802.81	0.3%	2.15	copy tree
4	893.38	1,119,341.18	0.3%	2.14	copy tree
5	890.22	1,123,313.28	0.1%	2.13	copy tree
1	390.42	2,561,319.35	0.3%	0.93	equality of tree
2	386.78	2,585,461.43	0.1%	0.93	equality of tree
3	393.43	2,541,776.30	0.7%	0.94	equality of tree
4	400.66	2,495,883.57	0.1%	0.96	equality of tree
5	399.51	2,503,047.44	0.4%	0.95	equality of tree