Support richer tuple unpacking

[tav]

I'm not sure if it's less efficient to do:

let mut (a, b, c, d) = (self.x[0], self.x[1], self.x[2], self.x[3]);

Than:

let mut a = self.x[0];
let mut b = self.x[1];
let mut c = self.x[2];
let mut d = self.x[3];

But I find the former a lot easier to type in some cases. It'd be nice to know if destructuring of this kind is a valid use of tuple unpacking in Rust.

It'd also be nice if the following was legal:

let (self.x[0], self.x[1], self.x[2], self.x[3]) = (a, b, c, d);

Right now the above dies with the following error:

expected `,` but found `.`

[nikomatsakis]

The first example should work, though it is indeed somewhat less efficient. The second does not work because what appears after let is a pattern, not just a list of lvalues.