[8_12] improve strategy of expanding string for performance

[jingkaimori]

see also #305

Performance

operator<< faster 2-3x than before

Before

ns/op	op/s	err%	total	benchmark
35.13	28,463,649.27	3.9%	0.22	construct string
2.66	376,488,992.00	1.7%	0.24	equality of string
17.68	56,561,986.49	1.0%	0.23	equality of larger string
4.14	241,610,858.14	0.9%	0.24	compare string
8.09	123,542,843.98	2.1%	0.24	compare larger string
18.23	54,847,647.47	0.6%	0.24	slice string
24.93	40,119,524.68	2.0%	0.24	slice string with larger range
31.80	31,447,902.44	0.9%	0.24	concat string
16.71	59,850,717.22	2.5%	0.24	append string
3.17	315,195,579.00	0.5%	0.24	is quoted

After

ns/op	op/s	err%	total	benchmark
33.35	29,984,690.73	2.4%	0.23	construct string
2.64	378,639,521.02	2.0%	0.24	equality of string
15.69	63,733,330.98	3.0%	0.23	equality of larger string
4.22	236,987,827.04	3.2%	0.25	compare string
8.50	117,694,065.06	3.1%	0.24	compare larger string
22.21	45,014,771.61	3.7%	0.23	slice string
25.88	38,644,124.66	0.3%	0.24	slice string with larger range
30.28	33,028,549.24	0.9%	0.24	concat string
6.81	146,814,049.22	3.7%	0.23	append string
3.20	312,586,648.76	0.7%	0.23	is quoted

[da-liii]

LGTM