Explore inline dyn for less indirection

[kpreid]

Our use of Vec<Arc<dyn Listener>> and the fact that Listener implementations usually have Weaks mean that most listeners are stored and invoked with double indirection (or triple if you count the Vec). Can we use an inline-boxed-trait-object type (or dyn*, experimentally?) so as to avoid the double indirection? objectionable is a library that claims to provide an InlineBox type which does what we need for that, but I have not tried actually using it.

Another strategy would be a "Vec of unsized elements" container, of which there are a couple experimental ones to be found:

• https://docs.rs/unsized-vec (nightly only)
• https://docs.rs/dyn_vec

However, using this would come at the cost of some awkward API (until such time as CoerceUnsized is stable) and we could no longer have dyn Listen (because the concrete type would have to be passed generically all the way to the Notifier all the time).

[kpreid]

The current state of unstable dyn* is that Rc, Arc, Weak cannot be used in it, and changing that is not quite as simple as adding the necessary impls to alloc. Filed rust-lang/rust#134591 for that.