Rename `loop` to `continue`

[brson]

Where most languages use continue to resume execution of a loop we reuse loop. We don't need to be breaking the mold here - change it to continue.

Nominating backcompat.

[erickt]

+1. I think we were being a bit too clever with loop.

[klutzy]

+1

[bstrie]

We'll also need to decide the fate of the current loop as while true. I rather like it, but I don't think it really warrants its own keyword.

[catamorphism]

Accepted for backwards-compatible