OpenSSL::PKey::EC::Group does not support 4-argument constructor

The ...EC::Group class should support a four-element constructor, as seen in several tests for ...EC::Point: https://github.com/ruby/ruby/blob/7f407e0240ebd41d9fb1ea1bbd15442ed8744b34/test/openssl/test_pkey_ec.rb#L351

I've got a broken attempt to implement that form coming in a PR, but I'm unsure how to map the OpenSSL calls to BC yet.
