Make create_text_context fn static and move it inside create_instance fn.

Make create_text_context fn static and move it inside create_instance fn.

We'd like to use the fid mempool for text_context that is stored
in the instance (we used cmd mempool before, so the order of
initialisation was not a matter, but now it is since we need to
create the fid mempool first which happens in create_instance fn).

The text_context initialisation is not needed anywhere outside the
create_instance fn so move it there.