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

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 :/

Concerns have been raised that including logging functionality makes it harder
to audit the implementation to ensure that no secret information is leaked. We
are therefore removing it from the master branch.

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

Add versions of olm_session_create_inbound and olm_session_matches_inbound which take the curve25519 identity key of the remote device we think the message is from as an additional argument

Clear the random buffers once they've been used. Add methods for clearing the account and session object once they've been used

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