sync::broadcast: create Sender::new

[marcospb19]

Closes #4958

[marcospb19]

I don't know how to fix the CI problem:

---- broadcast_channel_panic_caller stdout ----
thread 'broadcast_channel_panic_caller' panicked at 'assertion failed: `(left == right)`
  left: `"/home/runner/work/tokio/tokio/tokio/src/sync/broadcast.rs"`,
 right: `"tokio/tests/sync_panic.rs"`', tokio/tests/sync_panic.rs:22:5

[Darksonn]

The new_with_receiver_count method is fine, but I have some comments.

[Darksonn]

The two calls to this method should have a SAFETY comment that explains why the code is correct.

[marcospb19]

    // SAFETY: In the line below we are creating one extra receiver, so there will be 1 in total.

and

    // SAFETY: We don't create extra receivers, so there are 0.

[Darksonn]

Since this is an unsafe method, it should have a comment that explains what the safety requirements are.

[marcospb19]

+    /// Creates the sending-half of the [`broadcast`] channel, and provide the receiver count.
+    ///
+    /// See the documentation of [`broadcast::channel`] for more errors when calling this
+    /// function.
+    ///
+    /// # Safety:
+    ///
+    /// The caller must ensure that the amount of receivers for this Sender is correct before
+    /// the channel functionalities are used, the count is zero by default, as this function
+    /// does not create any receivers by itself.

[Darksonn]

I would move this to a real method since you're defining it twice.

[marcospb19]

Oops, I missed that Sender::receiver_count is 100% equivalent to what I was doing.

Updated.

[Darksonn]

Thanks, LGTM.