Non-blocking destructive bus

Hi!

I have a need for a non-blocking sender. When the bus is full, I'd like to overwrite oldest non-read elements (so readers might skip items if they're too slow) rather than blocking. `Bus::try_broadcast` is not working for me since I still need to send the new message when the bus is full.

Is it something this library might add?

Start by reading the existing Bus::try_broadcast behavior and the surrounding broadcast-channel API. Determine how a non-blocking destructive sender should interact with full buses and slow readers. Done means the requested behavior is defined and implemented with coverage for full and partially read buses.

Written by the indexing model from the issue text.