Make EVP_CIPHER opaque and add creator/destructor/accessor/writer functions

We follow the method used for EVP_MD.
Also, move all the internal EVP_CIPHER building macros from evp_locl.h
to evp_int.h.  This will benefit our builtin EVP_CIPHERs.

Reviewed-by: Rich Salz <rsalz@openssl.org>