rustc: Support irrefutable patterns in function arguments

[pcwalton]

r? @nikomatsakis

[pcwalton]

1. Good point, I'll update the tutorial when I push.
2. Yes, the plan is to convert all the code over. This addresses (3) too; we could forbid it, but I think it'd be faster to just convert all the old "drop" code over.
3. I thought about that, but the way to run the destructor is to just drop the last reference. There are many ways to do that.
4. It has to live in the same crate. We've talked about perhaps tightening it up to the same file; perhaps we should do that.

[nikomatsakis]

With respect to the drop trait: r+ although I think dtors ought to apply equally to enums/structs. In any case, I presume trans is coming later.

[pcwalton]

re-r? @nikomatsakis

[nikomatsakis]

r+ modulo the change to borrowck. Also, maybe add this as a compile-fail test:

pure fn call_first((x, y): (&fn(), &fn())) { 
    x(); //~ ERROR access to impure function prohibited in pure context
}
fn main() {}