[8_11] optimize performance of as_hexadecimal

[jingkaimori]

Performance

Before

complexityN	ns/op	op/s	err%	total	benchmark
1	16.13	61,983,083.42	1.1%	0.08	convert unsigned int to hexadecimal string
2	17.66	56,613,212.10	1.6%	0.09	convert unsigned int to hexadecimal string
3	18.61	53,742,458.03	0.4%	0.09	convert unsigned int to hexadecimal string
4	20.91	47,819,863.10	0.6%	0.10	convert unsigned int to hexadecimal string
5	24.17	41,368,070.45	0.4%	0.12	convert unsigned int to hexadecimal string

After

complexityN	ns/op	op/s	err%	total	benchmark
1	15.82	63,221,389.11	0.4%	0.08	convert unsigned int to hexadecimal string
2	15.76	63,449,120.53	1.6%	0.08	convert unsigned int to hexadecimal string
3	17.98	55,615,898.20	1.0%	0.09	convert unsigned int to hexadecimal string
4	17.53	57,043,784.30	2.8%	0.08	convert unsigned int to hexadecimal string
5	16.19	61,784,470.21	0.4%	0.08	convert unsigned int to hexadecimal string