The python that searches the header files for exports isn't smart
enough to know what is a comment and what isn't, so it picks this
up too and emscripten then complains about it being undefined.

also make some params const where possible

(cherry picked from commit c0a53ebaeda420f636698b99c393868d2bf8f905)

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.

This disrupts building at least with Visual Studio.

Signed-off-by: Alexey Rusakov <ktirf@users.sf.net>

olm_import_inbound_group_session, which reads the format written by
olm_export_inbound_group_session to initialise a group session.

A pair of functions which allow you to export the megolm keys for an inbound
group session, so that an application can save/restore them.

All the other methods clear their random inputs. This one needs to do the same,
to reduce the risk of the randomness being used elsewhere and leaking key info.

Applications can use the index to detect replays of the same message.

Document what it actually returns.

Add ed25519 keys to the inbound and outbound sessions, and use them to sign and
verify megolm messages.

We just stuff the ed25519 public key in alongside the megolm session key (and
add a version byte), to save adding more boilerplate to the JS/python/etc
layers.

Remove redundant args from some js funcs, and fix a comment typo

... so that I can use them from the group session bits.

I find myself wanting to know if an OlmSession is in the pre-key state or not,
to help debugging at the application level.

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.