Make Coroutine constructors' generic type arguments require 'static

[zetanumbers]

Solves #27 once and for all (most likely).

[Amanieu]

I think this also needs to be done for on_stack.

[zetanumbers]

I think this also needs to be done for on_stack.

I see no reason for this. The original issue was caused by violating std::thread::scope's assumption about references staying valid at least until function return. This in turn is caused by yielding from a function including from std::thread::scope's input closure, and then forgetting a coroutine would allow to invalidate such reference.

[loongs-zhang]

I think this also needs to be done for on_stack.

I see no reason for this. The original issue was caused by violating std::thread::scope's assumption about references staying valid at least until function return. This in turn is caused by yielding from a function including from std::thread::scope's input closure, and then forgetting a coroutine would allow to invalidate such reference.

The on_stack maybe used in coroutine.