[8_13] improve performance when calculating hash of string

[jingkaimori]

Performance

hash(string) is 110%-120% faster than before.

Before

ns/op	op/s	err%	total	benchmark
6.86	145,850,216.76	0.7%	0.24	hash of string
59.22	16,887,153.73	1.1%	0.24	hash of larger string

After

ns/op	op/s	err%	total	benchmark
6.22	160,754,437.19	0.3%	0.24	hash of string
50.64	19,747,047.60	0.2%	0.24	hash of larger string

[da-liii]

LGTM