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.

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

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