Add sugar to simplify mutable function arguments

[erickt]

While it's pretty easy to write a function that makes an argument mutable:

fn foo(x: ~int) {
    let x = move x;
    ...
}

It'd be nice to save a line here and there to add some sugar to express an argument is directly mutable, with something like:

fn foo(mut x: ~int) { ... }

[nikomatsakis]

+1

[bstrie]

I've stubbed my toe on this as well.

[nikomatsakis]

I started tagging some of the places in the code where this would be useful with FIXME(#3488)

[Dretch]

+1

[sanxiyn]

I am working on this.

[catamorphism]

Fixed: 54344c2

[sanxiyn]

Can places with FIXME be fixed, or does that require a snapshot?

[catamorphism]

@sanxiyn Yes, that will require a snapshot, because it has to be possible to compile the libraries with the stage0 compiler (the existing snapshot). But we are going to make a snapshot soon.