Optimise foreach on builtin::indexed

[leonerd]

Rather than generating an entire temporary list that is twice as big as the original array, instead set a flag on the OP_ITER that tells it to set one of the iteration variables to the current array index and use the same CXt_LOOP_ARY optimisastion that regular foreach over an array would use.

Currently this is a work-in-progress:

• Consider whether builtin::indexed LIST... should do the same optimisation
• Write perldelta

• This set of changes requires a perldelta entry, and it is not included but I'll write something before un-drafting

[leonerd]

This change does have the potential impact that, before it, any modifications to the iterated-on array that happen during the body of its own foreach loop would not get seen, whereas now they would. This is the same issue that single-variable foreach on a plain array already had, so it's not new. But it is a change for existing code that already calls this function.