lua: use an autofree wrapper instead of mp_lua_PITA

Advantages of this approach:

- All the resources are released right after the function ends
  regardless if it threw an error or not, without having to wait
  for GC.

- Simpler code.

- Simpler lua setup which most likely uses less memory allocation and
  as a result should be quicker, though it wasn't measured.

This commit adds the autofree wrapper and uses it where mp_lua_PITA
was used. It's not yet enforced at the C level, there are still
redundant talloc_free_children leftovers, and there are few more
places which could also use autofree. The next commits will address
those.