Import initial code for the SM2 cipher

Import initial code for the SM2 cipher

This adds the SM2 algorithm defined in the Chinese standards
GB/T 32918.1-2016, GB/T 32918.2-2016, GB/T 32918.3-2016,
GB/T 32918.4-2016 and GB/T 32918.5-2017.

This is an ISC licensed implementation contributed by Ribose.inc, based
on the same code that was contributed to OpenSSL by Jack Lloyd. The port
to LibreSSL was done by Ronald Tse and Nickolay Olshevsky.

Github PR #105

I made quite a few cleanup passes on this, but more is needed, some
of which will happen in-tree before this is linked to the build.

ok deraadt inoguchi (a long time ago), jsing