hash: use Go runtime's hash function

[alandonovan]

It uses AESENC instructions on amd64 and for strings of length >=12 starts to rapidly outperform the software implementation. It requires the linkname hack.

BenchmarkStringHash/hard-1-12         	300000000	         4.24 ns/op
BenchmarkStringHash/soft-1-12         	1000000000	         2.48 ns/op
BenchmarkStringHash/hard-2-12         	300000000	         4.88 ns/op
BenchmarkStringHash/soft-2-12         	500000000	         3.08 ns/op
BenchmarkStringHash/hard-4-12         	200000000	         6.19 ns/op
BenchmarkStringHash/soft-4-12         	300000000	         4.38 ns/op
BenchmarkStringHash/hard-8-12         	200000000	         7.58 ns/op
BenchmarkStringHash/soft-8-12         	300000000	         5.70 ns/op
BenchmarkStringHash/hard-16-12        	200000000	         8.49 ns/op
BenchmarkStringHash/soft-16-12        	100000000	        10.8 ns/op
BenchmarkStringHash/hard-32-12        	200000000	         8.68 ns/op
BenchmarkStringHash/soft-32-12        	50000000	        24.8 ns/op
BenchmarkStringHash/hard-64-12        	100000000	        10.5 ns/op
BenchmarkStringHash/soft-64-12        	30000000	        56.6 ns/op
BenchmarkStringHash/hard-128-12       	100000000	        15.3 ns/op
BenchmarkStringHash/soft-128-12       	10000000	       125 ns/op
BenchmarkStringHash/hard-256-12       	100000000	        17.7 ns/op
BenchmarkStringHash/soft-256-12       	 5000000	       267 ns/op
BenchmarkStringHash/hard-512-12       	50000000	        26.7 ns/op
BenchmarkStringHash/soft-512-12       	 3000000	       551 ns/op
BenchmarkStringHash/hard-1024-12      	30000000	        45.1 ns/op
BenchmarkStringHash/soft-1024-12      	 1000000	      1143 ns/op