The broadcast channel allows multiple senders to send messages to
multiple receivers, where each receiver receives messages starting from
when it subscribes. After all senders are dropped, the receivers will
continue to receive all waiting messages in the buffer and then receive
a `Closed` error.

To mark that a channel has closed, it stores two closed flags, one on
the channel level and another in the buffer slot *after* the last used
slot (this may also be the earliest entry being kept for lagged
receivers, see tokio-rs#2425).

However, we don't need both closed flags, keeping the channel level
closed flag is sufficient.

Without the slot level closed flag, each receiver receives each message
until it is up to date and for that receiver the channel is empty. Then,
the actual return message is chosen depending on the channel level
closed flag; if the channel is NOT closed, then `Empty` is returned, if
the channel is closed then `Closed` is returned instead.

With the modified logic, there is no longer a need to append a closed
token to the internal buffer (by setting the slot level closed flag on
the next slot). This fixes the off by one error described in tokio-rs#4814,
which caused a receiver which was created after the channel was already
closed to get `Empty` from `try_recv` (or hang forever when calling
`recv`) instead of receiving `Closed`.

As a bonus, we save a single `bool` on each buffer slot.

Refs: tokio-rs#4814

replace unsafe call with `UnsafeCell::new` as the unsafe isn't necessary.

Co-authored-by: Alice Ryhl <aliceryhl@google.com>