Returns the registry table (see Registry).
Lua provides a registry,
a pre-defined table that can be used by any C code to
store whatever Lua value it needs to store.
This table is always located at pseudo-index
Any C library can store data into this table,
but it should take care to choose keys different from those used
by other libraries, to avoid collisions.
Typically, you should use as key a string containing your library name
or a light userdata with the address of a C object in your code.
The integer keys in the registry are used by the reference mechanism,
implemented by the auxiliary library,
and therefore should not be used for other purposes.