[8_12] improve strategy of expanding string for performance

[jingkaimori]

This pr ensure the layout consistency between array and string, by make member pointer appears at the same position of class, so that union inside tree will not be broken.

#331

class string{
int n;
int a_N;
char* a;
//...
}

template<class T>
class array<T>{
int n;
T* a;
//...
}

this pr

class string{
int n;
char* a;
int a_N;
//...
}

template<class T>
class array<T>{
int n;
T* a;
//...
}

Performance

Before

ns/op	op/s	err%	total	benchmark
28.76	34,766,499.89	1.9%	0.23	construct string
2.15	465,519,808.15	1.1%	0.24	equality of string
12.29	81,372,418.85	3.4%	0.24	equality of larger string
3.67	272,834,596.28	1.0%	0.24	compare string
6.77	147,721,592.92	1.4%	0.24	compare larger string
15.25	65,576,665.94	0.2%	0.24	slice string
25.84	38,706,819.98	0.6%	0.24	slice string with larger range
26.77	37,362,109.49	1.9%	0.24	concat string
14.44	69,248,803.63	2.7%	0.23	append string
3.95	253,250,191.20	0.7%	0.24	hash of string
31.15	32,099,101.32	0.5%	0.24	hash of larger string
2.45	408,268,298.94	1.1%	0.24	is quoted

After

ns/op	op/s	err%	total	benchmark
33.28	30,048,811.93	0.5%	0.24	construct string
2.10	476,358,528.82	1.9%	0.24	equality of string
12.61	79,290,902.83	3.3%	0.24	equality of larger string
3.69	271,309,979.88	1.5%	0.24	compare string
6.98	143,291,461.49	1.6%	0.24	compare larger string
15.86	63,070,424.48	1.1%	0.23	slice string
22.09	45,270,914.17	0.3%	0.24	slice string with larger range
25.83	38,707,374.61	2.1%	0.24	concat string
5.74	174,232,028.58	2.3%	0.22	append string
3.42	292,593,737.92	0.6%	0.24	hash of string
33.60	29,760,571.57	1.2%	0.24	hash of larger string
2.35	424,756,682.83	0.8%	0.24	is quoted

[da-liii]

bench的结果在Pull Request的描述里面贴一下吧

[da-liii]

依据PR里面的描述，前三项的性能退化了。

[da-liii]

在第三次提交下，性能应该有改善，需要一个新的性能对比，来判断性能上是否提升或者降低。

[jingkaimori]

需要一个新的性能对比，来判断性能上是否提升或者降低。

已更新在pr的说明中

[da-liii]

slice/concat/append 变快了，别的基本上差不多，略有变慢

[da-liii]

这是之前的代码：30170ac

[da-liii]

在PR描述或者代码里面写一下，这个pr和 #331 有什么区别，如何做到避免内存问题的

[da-liii]

LGTM