Data races in generator

The Generator type is an iterable which uses a generator function that yields
values. In affected versions of the crate, the provided function yielding values
had no Send bounds despite the Generator itself implementing Send.

The generator function lacking a Send bound means that types that are
dangerous to send across threads such as Rc could be sent as part of a
generator, potentially leading to data races.

This flaw was fixed in commit f7d120a3b
by enforcing that the generator function be bound by Send.