Putting the session_id inside the packed message body makes it hard to extract
so that we can decide which session to use. We don't think there is any
advantage to having thes sesion_id protected by the HMACs, so we're going to
move it to the JSON framing.

sha256.c and aes.c contain conflicting declarations, so we need to compile them
as separate units. This requires a bit more Makefile-shuffling; the build
directory now includes 'src' or 'lib' as appropriate, and we just mkdir -p
before each compilation.

There was some slightly overcomplex logic designed to save a couple of hash
operations when R(0) and R(1) were advanced, but the extra code was hard to
understand and didn't save much.

We no longer need to keep track of intermediate values of the counter, which
means we can update it much more easily.

These were left over from when rehash_part did a bunch of logging.

I want to use test_olm.sh on jenkins, which lacks jq. Hence, add support to
olm.main to get id and one-time keys individually.

think this got lost when mark added separate release/debug builds

Factor the actual message encoding/decoding and encrypting/decrypting out to
separate functions from the top-level functions which do the base64-wrangling.

This is particularly helpful in the 'outbound' code-path where the offsets
required to allow room to base64-encode make the flow hard to see when it's all inline.

Initialise megolm_cipher via the preprocessor macro, instead of with a
function.

Some of the crypto libs rely on UINT64_C, which in glibc 2.17 and earlier was
not defined for C++ code (see
https://sourceware.org/bugzilla/show_bug.cgi?id=15366).

memset is at risk of being optimised away, so use _olm_unset instead.

The behaviour when casting from a uint32_t which has overflowed (so has the top
bit set) to int32_t is implementation-defined, so let's avoid relying on it.

Make names (of session_key and message_index) more consistent.

Use our own protobuf tags rather than trying to piggyback on the one-to-one
structure.

Includes creation of inbound sessions, etc

We need to be able to inspect an outbound session so that we can tell our peer
how to set up an inbound session.

We don't need to have all of the top-level pickling functions in olm.cpp;
factor out the utilities to support it to pickle_encoding.cpp (and make sure
that they have plain-C bindings).

I want to be able to use this functionality from elsewhere, so factor it out to
its own file.

Replace the init-static-var dance with some preprocessor macros

We never delete a cipher, and the destruct op is empty, so it's a bit pointless

Give a load of internal symbols "_olm_" prefixes. This better delineates the
public and private interfaces in the module, and helps avoid internal symbols
leaking out and possibly being abused.