Adds the OLM_PICKLE_EXTRA_DATA error code. We fail with this code when
the pickle object looks right except for some unexpected trailing bytes
which we didn't process.
Change interface to allow the app to get the private part of the
key and instantiate a decryption object from just the private part
of the key.
Changes the function generating a key from random bytes to be
initialising a key with a private key (because it's exactly the
same thing). Exports & imports private key parts as ArrayBuffer at
JS level rather than base64 assuming we are moving that way in
general.
