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.

Should have been in the previous commit :/

These seem to be the wrong way around.

So that we can build everything together.

Split the python wrappers into separate modules