destructuring definitions

[jeisner]

Would like to allow "destructuring definitions" like the first line below:

[baz,[bing,quux]] += temp.
temp = [5,[6,7]].

or

pair[baz,pair[bing,quux]] += temp.
temp = pair[5,pair[6,7]].

The details and corner cases are currently written up on an email thread starting on 7/2/13, which should possibly be copied here.

I wrote there:

I like destructuring definitions even if we find a way to get away without them for argmax= and argmin=. They seem like a very natural parallel to unification. But they might not be as high-priority a feature if we don't need them for argmax= and argmin=.

See #29 for discussion of the argmax= and argmin= stuff.