Hello! This crate is very neat. It would be very helpful for metrics if mpsc::Sender and mpsc::Receiver channel handles exposed a way to determine the capacity and remaining slots. The ThingBuf queue has a remaining() and a capacity() method, so it seems odd that Sender or Receiver don't. I'm open to writing up a PR to add this, but I noticed that the implementation of Sender and Receiver was a bit different than ThingBuf -- is there any reason these weren't exposed? From a quick glance it seems like they would be easy to add to Sender and Receiver by just using the len() and capacity() methods from self.inner.core.core