Return the message index when decrypting group messages.

Add notes on limitations to megolm spec

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

We're standardising on 'one-time keys' as a term for the thing that Bob uploads
for prekey messages.

Fix a buffer bounds check when decoding group messages

Add a README for the fuzzers

Fixes a segfault when a group message had exactly the length of the mac +
signature.

Also tweak skipping of unknown tags to avoid an extra trip around the loop.

These missing signatures were causing OSX to truncate 64-bit pointers
to 32-bit pointers when calling the missing methods, causing segfaults

Signed-off-by: Emmanuel Gil Peyrot <linkmauve@linkmauve.fr>

Split ratchet algorithm out to a separate section.

Also clean up some phrasing and correct a typo or two.

Fix jenkins.sh

Update comment on olm_account_one_time_keys

Document what it actually returns.

Remove the messsage index from olm_init_inbound_group_session