Resize the capacity of bounded channels

[Frando]

It would be neat if bounded channels could be resized, to increase or decrease the allocated capacity, like Vec::shrink_to and Vec::reserve_exact.

[zesterer]

I would indeed be neat, but potentially annoying to implement properly. Sadly, I don't currently have the time to implement substantial features like this.

[rakbladsvalsen]

This issue is similar to https://github.com/zesterer/flume/pull/148, where you basically can end up with memory leaks due to the way flume internally works. Not quite the same issue, but memory leaks can't be avoided because there's no shrink_to_fit in any channel flavor.

I believe the "proper" implementation would be to:

1. provide proxy methods to the internal queue's shrink_to_fit and reserve_exact methods, among others, or
2. provide something like alter_queue(Fn(&mut VecDeque))/get_queue_mut() -> &mut VecDeque so that the user is free to do whatever they want, including, but not limited to shrink_to_fit, reserve_exact, etc. Although this could potentially cause bugs (?), I don't think it's that bad. I've seen libraries like reqwest use weird fn names to convey the fact that, by using them, they might cause issues or problems (example).

Resizing bounded channels might be actually hard to implement because you'd need a way to change how the internal Channel works (its capacity isn't stored in a mutable/atomic integer), but you can probably achieve whatever you're trying to do with wrapped unbounded channels + the stuff I implemented in my PR.