[8_8] Optimize comparison between string

[jingkaimori]

Optimize comparison of two string by dictionary order. After this work, performance of large string comparison is improved twice, however short one is not improved.

Performance

Before

ns/op	op/s	err%	total	benchmark
22.16	45,121,128.76	0.2%	0.01	compare string
145.29	6,882,788.16	1.5%	0.01	compare larger string

After

ns/op	op/s	err%	total	benchmark
23.05	43,390,782.34	4.6%	0.01	compare string
68.25	14,651,940.51	1.5%	0.01	compare larger string