Before, we needed to always pack an entire msgpack_rpc Object to a continous memory buffer before sending it out to a channel. But this is generally wasteful. it is better to just flush whatever is in the buffer and then continue packing to a new buffer. This is also done for the UI event packer where there are some extra logic to "finish" of an existing batch of nevents/ncalls. This doesn't really stop us from flushing the buffer, just that we need to update the state machine accordingly so the next call to prepare_call() always will start with a new event (even though the buffer might contain overflow data from a large event).
Then we can just load metadata in C as a single msgpack blob. Which also can be used directly as binarly data, instead of first unpacking all the functions and ui_events metadata to immediately pack it again, which was a bit of a silly walk (and one extra usecase of `msgpack_rpc_from_object` which will get yak shaved in the next PR)
As only a few API functions make use of explicit freeing of the return value, make it opt-in instead. The arena is always present under the hood, so `Arena *arena` arg now doesn't mean anything other than getting access to this arena. Also it is in principle possible to return an allocated value while still using the arena as scratch space for other stuff (unlikely, but there no reason to not allow it).
Note: kSDItemHeader is something is _written_ by nvim in the shada file to identify it for debugging purposes outside of nvim. But this data wasn't ever used by neovim after reading the file back, So I removed the parsing of it for now.
Note: this contains two _temporary_ changes which can be reverted once the Arena vs no-Arena distinction in API wrappers has been removed. Both nlua_push_Object and object_to_vim_take_luaref() has been changed to take the object argument as a pointer. This is not going to be necessary once these are only used with arena (or not at all) allocated Objects. The object_to_vim() variant which leaves luaref untouched might need to stay for a little longer.
and for return value of nlua_exec/nlua_call_ref, as this uses the same family of functions. NB: the handling of luaref:s is a bit of a mess. add api_luarefs_free_XX functions as a stop-gap as refactoring luarefs is a can of worms for another PR:s. as a minor feature/bug-fix, nvim_buf_call and nvim_win_call now preserves arbitrary return values.