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.

Also adds tests to ensure this is working.

as some compilers don't handle that

make olm_pickle_* return the lengths of the base64-encoded pickles, rather than
the raw pickle. (From the application's POV, the format of the pickle is
opaque: it doesn't even know that it is base64-encoded. So returning the length
of the raw pickle is particularly unhelpful.)

Ed25519 private keys, it turns out, have 64 bytes, not 32.

We were previously generating only 32 bytes (which is all that is required to
generate the public key), and then using the public key as the upper 32 bytes
when generating the per-message session key. This meant that everything
appeared to work, but the security of the private key was severely compromised.

By way of fixes:

 * Use the correct algorithm for generating the Ed25519 private key, and store
   all 512 bits of it.

 * Update the account pickle format and refuse to load the old format (since we
   should consider it compromised).

 * Bump the library version, and add a function to retrieve the library
   version, so that applications can verify that they are linked against a
   fixed version of the library.

 * Remove the curve25519_{sign, verify} functions which were unused and of
   dubious quality.

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