Method of turning result of pkey:getParameters() back into a key

[Zash]

This would allow loading a serialized JWK structure back into a usable form.

[wahern]

I've committed support for a pkey:setParameters method in HEAD. I still need to modify pkey.new so it can accept a table of initializers rather than just a PEM or DER string. Currently you can only generate a dummy key and then replace the parameters.

For EC keys :getParameters now returns an additional field, group, a new userdata type binding the EC_GROUP object.

I might not be able to finish for a couple of weeks, but hopefully what's committed should be enough as-is.