index tag method

["John Belmonte" <jvb@...>]

The index tag method is a sub-case of gettable and can be removed, reducing
the complexity of tag methods and slimming down luaV_gettable in the virtual
machine.  If the user wants to treat a previously nil  (or any other value
or type for that matter) index as a special case they can do it with
gettable, so why go as far as having this in the VM which slows down all
table reads?

At least for Sol this would make a lot of sense since 1) tag method
backwards compatibility was already tossed and 2) global reads go through
normal table access so the VM's table read is even more of a workhorse than
Lua.

-John