De-implement looping over several elements per iteration

[masak]

This currently works in 007:

$ bin/007 -e='for [1, 2, 3, 4] -> x, y { say(str(x) ~ " and " ~ str(y)) }'
1 and 2
3 and 4

I suggest we remove this feature. Naming a single variable as the loop variable will still be allowed.

Why? A couple of reasons:

• The feature has not seen much use. In examples/ we currently have 3 for loops. All of them name one variable.
• As we near v1.0, I'm getting increasingly aware of what features we're going to need to support long-term.
• It's simpler code to do just one variable.
• Making it 0-or-1 variables would make it identical to how the if statement works.
• It's a smaller surface area to expose to APIs and the like to do just one variable.
• There's the ever-present little glitch wherein if the elements don't come out even in the end, you get None for some of them. Not a big problem, just slightly inelegant.
• The way I'd want this to work, on reflection, is using subsignature matching. Here's some hypothetical code: for [[1, 2], [3, 4]] -> [x, y] { say(str(x) ~ " and " ~ str(y)) }. Under something like Implement type checking, orthogonally to everything else #33, we could even get nice structural checking for that. We couldn't with the current implementation.

Lastly, .map doesn't do that trick in 007 (and I don't think it should):

$ bin/007 -e='[1, 2, 3, 4].map(sub (e1, e2) { say(e1 ~ " and " ~ e2) })'
Sub with 2 parameters called with 1 arguments