Array subtype is conceptually simplest

[Dirk Laurie <dirk.laurie@...>]

* Every Lua 5.2 program you ever wrote in which somewhere the argument
to 'ipairs' is a table with an __index metamethod, has a chance of
being broken in Lua 5.3, because that metamethod _will_ be invoked in
a way that probably is not the reason why you supplied it. It has
happened three times to me, so far, and the way in which the program
fails has on each occasion been so different that I have not really
learnt by experience. Each time, it takes me several hours of
debugging time.

* Lua 6.0 will very probably have some kind of mechanism to address
the holes-in-tables problem that haunts the table library. With the
present ipairs semantics, that mechanism may well re-break programs
that have been fixed for Lua 5.3.

The 5.4-work1 approach solved a too-ambitious problem: allowing a
significant nil in any field of a table. To fix the problems that new
users have with the table library, it is only necessary to allow that
in the range 1..#tbl.

An array subtype would solve that problem.

1. Own __len, implicit in the subtype, if you set your own __len it is
no longer an array.
2. ipairs iterates from 1 to __len, table functions use __len.
3. Value returned by __len is fixed when the table is created and
can't be changed by assignment, only by functions in the table library
(insert, remove, setlen).