swap arg order of vec::init_fn

[erickt]

This allows for the block syntax sugar.

[nikomatsakis]

Argh, I thought I got all of these!

[gmfawcett]

Pull request #1559 . (Is there a way to formally link a pull request to an issue? I couldn't find one...)

[erickt]

@gmfawcett: github automatically links a "#1352" to an issue. If you look at the issue url #1352, you'll see it added a reference for you automatically.

[gmfawcett]

@erickt : thanks, I see it linked to my commit, but it's not clear (to me) from context that there's a pull request associated with it.