Add sm2/3/4 support

[fengyc]

Add sm2/3/4 support, comments are welcome :-)

[mingjunyang]

now, sm2/3/4 curve very important and helpfull for some CN developer. I have started testing this sm2 branch in my some project.

Hope it can be merged into the master branch soon.

thanks！

[jiayang-zheng]

Is there any plan to merge SM2/3/4 encryption in main branch please? @npmccallum
It may help a lot if these encryption algorithms are supported in openssl rust crate.

Thanks!

[npmccallum]

@jiayang-zheng I don't have commit privileges on this repo.

[jiayang-zheng]

@npmccallum Thank you all the same :P
Hope this pr could be merged into the master branch soon :-)

[jiayang-zheng]

Since SM2/3/4 encryption is more and more popularized and important - especially in China, is there any plan to support these encryption algorithms please? @sfackler

Thanks!