Allow tuples and other irrefutable patterns in parameter specifications

[nikomatsakis]

Currently we can write

for (a, b) in vec::zip(as, bs) { ... }

but not

vec::iter(vec::zip(as, bs)) { |(a, b)| ... }

nor

fn proc((a: A, b: B)) { ... }
vec::iter(vec::zip(as, bs), proc)

It would be nice to have these irrefutable patterns work in parameter specifications, if not in all functions then at least in Ruby-like blocks.

[marijnh]

I think this is a good idea. It was agreed on to support this during the Toronto meeting, but when I was implementing destructuring bindings I somehow forgot about it. There are some awkward corners to be decided on (we allow param names in fn types -- will we also allow pointless patterns there?), but on the whole this should be straightforward to implement. (Though a lot of code will have to be touched.)

[erickt]

Ack! I think I closed this by mistake. However, I think this is a dup of #1881, so I'm going to leave it closed.