correction in macro for hard tests + type in comment

As the GC is not reentrant, finalizers should not be able to invoke it.

According to ISO C, "making a function an inline function suggests that
calls to the function be as fast as possible." (Not available in C89.)

'index2value' accepts pseudo-indices also when called from a Lua
function, through a hook.

The existence of 'lua_closeslot' is no reason for lua_pop not to close
to-be-closed variables too.  It is too error-prone for lua_pop not to
close tbc variables being popped from the stack.

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.

Long time without using HARDSTACKTESTS...

Comments, code details, identation.

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.