Add notes on limitations to megolm spec

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

Add a olm_inbound_group_session_id method