Add custom API to prevent Lua Objects from being garbage collected.
This is intended to for Ardour LuaBridge bindings (~1MB Objects:
tables, functions and userdata).
The bindings are persistent and the gc can skip them in mark & sweep
phases. This is a significant performance improvement for garbage
collection.
Note. The next version of Lua (5.4) will come with a generational-gc
rather than an incremental, so extending the API at this point in time
is acceptable.
