OLMKit:  New pickle version using a pickle key provided externally

See merge request matrix-org/olm!17

Improve ObjC wrappers so that they can use a pickle key provided by the olm lib user.

This new behavior is optional to not break existing usage.

It is retro compatible and use pickle versioning already in place. 
Existing key will be unpickled with pickle v1 and pickled with pickle v2 if an external pickle key is provided. 

Update index.d.ts; specify PRIVATE_KEY_LENGTH const export

See merge request matrix-org/olm!15

remove other_key checks from Python binding since it's done in C now

See merge request matrix-org/olm!14

Also make olm_sas_generate_bytes fail if their key wasn't set.

so that we're under GitLab's 1000 character limit for rendering maths

add support for fallback keys

See merge request matrix-org/olm!13

Add 4 by default, and make sure OT keys are not reused.