We were using olm::KEY_LENGTH for everything under the sun which happened to be
32 bytes long, and making a bunch of assumptions in the process. Create a bunch
of new constants (as C #defines rather than C++ consts so that I can use them
in another forthcoming refactor).
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.
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.
