Move-out-of-raw-pointer in vec::shift

[graydon]

Comment says 'this isn't legal, per se..' which is cute but could do with a bit more explanation of why it's even working.

[nikomatsakis]

Actually I think that's legal now. I recently updated the rules to allow
moves out of unsafe pointers (it's very useful sometimes!)

We were "allowing" it before just because we weren't treating let x <- y
as a move.

[eholk]

It's legal now, so I'll remove the comment.