RustCrypto `sm2` crate

In the @RustCrypto org we're interested in developing an `sm2` crate using our generic implementation of prime order elliptic curve arithmetic in the `primeorder` crate:

https://github.com/RustCrypto/elliptic-curves/tree/master/sm2

Before working on that, I wanted to make sure it's okay if we use the `sm2` crate for this.

@andelf I saw you gave ownership of the `sm2` to @newpavlov but I just wanted to double check it would be okay if we go ahead and publish a new crate under that name before we proceed.