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

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

Thanks to Dominic Fischer for spotting.

instead of having a static one, as that could end up taking up a
lot of memory if your app keeps olm sessions hanging about.

As a way to dump the state of an olm session, ie. the chain indicies,
so we can debug why olm sessions break and get out of sync.

because older versions of emscripten don't support the length argument to
UTF8ToString.

The Pointer_stringify() function is deprecated and has a couple of
issues because it tries to guess the encoding of the buffer. In some
cases it can ignore the length parameter which could end up in
inconsistencies.

Switch to UTF8toString() that takes a length parameter and respects,
that way we don't need to allocate an additional byte for a NULL byte.