refactor(metadata): generate all metadata in lua

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)

src/nvim/generators/gen_api_ui_events.lua

@@ -204,7 +204,5 @@ for _, ev in ipairs(events) do
   end
 end
 
-local packed = mpack.encode(exported_events)
-local dump_bin_array = require('generators.dump_bin_array')
-dump_bin_array(metadata_output, 'ui_events_metadata', packed)
+metadata_output:write(mpack.encode(exported_events))
 metadata_output:close()