Yielding in a __close metamethod called when returning vararg results
changes the top and so messes up the number of returned values.

New module easies the inclusion of GC tracing in individual test files.

When, inside a coroutine, a C function with to-be-closed slots return,
the corresponding metamethods can yield. ('__close' metamethods called
through 'lua_closeslot' still cannot yield, as there is no continuation
to go when resuming.)

To-be-closed variables are linked in their own list, embedded into the
stack elements. (Due to alignment, this information does not change
the size of the stack elements in most architectures.)  This new list
does not produce garbage and avoids memory errors when creating tbc
variables.

The return hook should be called only after closing variables (which
are still part of the function). C functions were calling the hook
before the metamethods.

New field 'lua_State.ptbc' keeps to-be-closed variable until its
upvalue is created, so that it can be closed in case of a
memory-allocation error.

Completes commit b07fc10e91a. '__close' metamethods can yield even
when they are being called due to an error. '__close' metamethods from
C functions are still not allowed to yield.

Initial implementation to allow yields inside '__close' metamethods.
This current version still does not allow a '__close' metamethod
to yield when called due to an error. '__close' metamethods from
C functions also are not allowed to yield.