Only allow *two* or more items per collection of plans

[lvh]

Right now, the schema prohibits empty plans, because they're nonsensical:

[#{} a]

Not doing anything in any order is... strange, to say the least; so it makes more sense to complain rather than to accept it, because that doesn't look like something the user actually meant.

However, colls of a single element are also allowed, and they don't make any more sense:

[#{a} b]

First do a in any order, then do b? Yeah, that's just "first do a, then do b", which should canonically be spelled:

[a b]

So, maybe icecap should also reject 1-elem colls.