Add support for labeled while loops.
Fix formatting, update copyright dates
auto merge of #16190 : Pythoner6/rust/labeled-while-loop, r=alexcrichton

Fixes #12643

> Say!
> I like labelled breaks/continues!

I will use them with a `for` loop.
And I will use with a `loop` loop.
Say! I will use them ANYWHERE!
… _even_ in a `while` loop.

Because they're now supported there.