The call to 'luaL_newmetatable' in 'newbox' can leave an incomplete
metatable in the registry, if 'luaL_setfuncs' raises a memory error.

In 'luaV_finishset', there is an update on a table that is a field on
another table. If the first table is the same as the one with the field
(e.g., after 't.__newindex = t'), the update can change the value on
that field (e.g., there may be a collision and the field is moved, or
the field being updated is '__newindex' itself). After that, the
barrier is called with the table stored in that field, which is not
the correct table anymore.

It makes loop variables read-only, but it does not restore the old
semantics, so it is not helpful for compatibility.

(plus detail in the manual)

The union may have alignment requirements stricter than some of its
members. Some checking tools (e.g., gcc with options -fsanitize) can
then complain that the result of a cast from pointer to member to
pointer to the union is misaligned.

- Some api_checknelems changed to the more restrict api_checkpop.
- Added a class to the html for APIs in the manual.
- Comments and manual.

A close instruction is still inside the scope of the variables it is
closing. The extra close in a repeat-until (to close variables before
repeating the loop) was being coded outside that scope.