Keeping track of the chain index is a useful thing to do, but is only required
if we've enabled diagnostics. Extend the session pickle format to make a space
for it, so that pickles can be transferred between the logging_enabled branch
and the master branch without loss of information.

Also add some tests for session pickling which explicitly check that we can
unpickle both formats of pickle.

This was introduced when I was experimenting with support for logging progress
in Olm. That is now relegated to the logging_enabled branch, so this should
probably be removed.

This also fixes the incompatibility of session pickles from the current master
branch with those from olm 0.1.0.

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.

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

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

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.

Replace hard coded references to the 32-byte key length with a constant, add utilities for copying data to and from fixed sized arrays

Initialise the length fields of the reader struct in decode_message, even if the message is invalid, fixes a crash where the message was too short

Add a test of olm which uses malloc to create it's buffers so that it is possible to check of out of bounds reads and writes using valgrind

Fix bug in list where the wrong value was copied if an item was inserted at the beinging of the list

Output simpler JSON for the account keys, don't sign the JSON but instead provide a olm_account_sign method so that the user of the library can sign the JSON themselves

Rename axolotlpp as olm to avoid confusion with Axolotl-the-spec and Axolotl-the-OWS-libraries at moxie's request