While building a new prototype, the GC needs barriers for every object
(strings and nested prototypes) that is attached to the new prototype.

The parser were mixing compiler indices of variables with stack indices,
so that when a to-be-closed variable was used inside the scope of
compile-time constants (which may be optimized away), it might be closed
in the wrong place. (See new tests for examples.)

Besides fixing the bugs, this commit also changed comments and variable
names to avoid that kind of confusion and added tests.

The code should not compute an instruction address before checking that
it exists. (Virtually no machine complains of computing an invalid
address, as long as the address is not used, but for ISO C that is
undefined behavior.)

This function was computing invalid instruction addresses when the
expression was not a multi-return instruction. (Virtually all machines
don't raise errors when computing an invalid address, as long as the
address is not accessed, but this computation is undefined behavior in
ISO C.)

Fixed some minor issues from the feedback for 5.4-beta rc1.

Attributes changed to posfixed ('x <const>', instead of '<const> x'),
and "toclose" renamed to "close". Posfixed attributes seem to make it
clearer that it applies to only one variable when there are multiple
variables.

A reference to a 'Vardesc*' (as done by 'localstat') can be
invalidated by the creation of any new variable.

- OP_NEWTABLE can use 'ra + 1' to set top (instead of ci->top);
- OP_CLOSE doesn't need to set top ('Protect' already does that);
- OP_TFORCALL must use 'ProtectNT', to preserve the top already set.
  (That was a small bug, because iterators could be called with
   extra parameters besides the state and the control variable.)
- Comments and an extra test for the bug in previous item.

String literal expressions have their own kind VKSTR, instead of the
generic VK. This allows strings to "cross" functions without entering
their constant tables (e.g., if they are used only by some nested
function).

Opcodes OP_NEWTABLE and OP_SETLIST use the same representation to
store the size of the array part of a table. This new representation
can go up to 2^33 (8 + 25 bits).