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.
This makes more sense, because "generate" implies *running*
a generator, not *returning* from a generator. That name was
a vestige of the time when libfringe only really provided a .next()
implementation, which made marginally more sense together with
.generate(), but it doesn't anymore when we have .resume().
