That will allow to change pointers to offsets while reallocating
the stack.

'luaD_growstack' already checks that. This commit also fixes an
internal bug in 'luaD_growstack': a large 'n' could cause an arithmetic
overflow when computing 'needed'.

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.