Require stack to be 'static

[zetanumbers]

Fixes #43

[zetanumbers]

Unfortunately changes to the coroutine_call benchmark slowed it down by around +108% of cycles and +69% of time. Should I add with_stack_unchecked and use it from the benchmark? Not sure what exact safety rules would I write for it.

[Amanieu]

Maybe we should have a with_scoped_stack constructor that takes a non-'static stack and allows you to use it only within a closure. This should avoid the issue with forgetting a coroutine.