Remove the messsage index from olm_init_inbound_group_session

Add a olm_inbound_group_session_id method

Use the ed25519 public key as the group session id.

Some clients expect the session id to be globally unique,
so allowing the end devices to pick the session id will cause
problems.

Include the current ratchet index with the initial keys, this decreases
the risk that the client will supply the wrong index causing problems.

Sign the initial keys with the ratchet ed25519 key, this reduces the
risk of a client claiming a session that they didn't create.

Prepare changelog for v1.2.0

Sign megolm messages

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

Convert ed25519 pickling functions to C

Fix error handling for group sessions

Fix a couple of places where we were using the wrong method to get the last
error.

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

... because OSX doesn't support it.

Convert crypto.hh into C-compatible interface

OlmSession.has_received_message

Fix megolm decryption of UTF-8

Create new constants for key lengths, etc

Repeat the fix from b10f90d for megolm messages.

It turns out that the 'length' argument to 'Pointer_stringify' doesn't work if
the input includes characters >= 128.

Rather than try to figure out which methods can return UTF-8, and which always
return plain ascii, replace all uses of Pointer_stringify with a 'length'
argument with the version that expects a NULL-terminated input, and extend the
buffer by a byte to allow space for a null-terminator.

In the case of decrypt, we need to add the null ourself.

Fixes https://github.com/vector-im/vector-web/issues/2078.

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