Don't implement Send at all

[edef1c]

On hosted platforms, libstd allows safely borrowing values from TLS with 'thread lifetime and without a Sync bound. As a result, we can't guarantee that sending a generator across threads won't create dangling references or data races. In freestanding environments, the notion of thread-safety is likely to be defined by the consumer of libfringe, so our Send bounds and implementation are unlikely to be meaningful anyway.

[Amanieu]

More specifically, we can only make a generator Send if everything on the generator stack is Send. Since we have no practical way of enforcing that, we must be conservative and not make generators Send.

[whitequark]

Wait, we don't have a 'thread lifetime: https://github.com/rust-lang/rfcs/pull/1705#issuecomment-260061724.

[edef1c]

@whitequark The actual lifetime of TLS memory is (at most) 'thread, but that's indeed not expressible in the type system.