SM2 encryption Benchmark

P-256/SM2 (No tuning)
goos: windows
goarch: amd64
pkg: gmsm/sm2
BenchmarkLessThan32-6   	     210	   5665861 ns/op	   0.01 MB/s	 2601864 B/op	   27725 allocs/op
PASS
ok  	gmsm/sm2	5.629s


P-256
goos: windows
goarch: amd64
pkg: gmsm/sm2
BenchmarkMoreThan32-6   	   13656	     86252 ns/op	    3141 B/op	      50 allocs/op
PASS
ok  	gmsm/sm2	4.139s