evp cipher/digest - add alternative to init-update-final interface

libcrypto's interface for ciphers and digests implements a flexible
init-update(s)-final calling sequence that supports streaming of
arbitrary sized message chunks.

Said flexibility comes at a price in the "non-streaming" case: The
operation must be "artificially" split between update/final. This
leads to more functions than necessary needing to be called to
process a single paket (user errors). It is also a small paket
performance problem for (possibly engine provided) hardware
implementations for which it enforces a superfluous call to a
coprocessor or adapter.

libssl currently solves the problem, e.g for tls 1.2 aes-gcm record
layer encryption by passing additional context information via the
control interface and calling EVP_Cipher (undocumented, no engine
support. The analoguously named, undocumented EVP_Digest is just an
init-update-final wrapper). The same would be possible for tls 1.3
pakets (it is currently implemented using init-update-final and
performs worse than tls 1.2 record encryption on some s390 hardware).

I would suggest to add (engine supported) interfaces that can process a
paket with 2 calls (i.e. init-enc/dec/hash), at least for crypto
primitives that are often used in a non-streaming context, like aead
constructions in modern tls (This would also make it possible to move
tls specific code like nonce setup to libssl. Such interfaces already
exist in boringssl[1] and libressl[2]).

What do you think ?

Best,
Patrick

[1] 
https://commondatastorage.googleapis.com/chromium-boringssl-docs/aead.h.html
[2] http://man.openbsd.org/EVP_AEAD_CTX_init
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