Changes a couple of function signatures to return `Completion`. Also, I
changed `Completion` to be internally `Arc` to abstract the `Arc`
implementation detail, and to be able to attach a `#[must_use]` to the
`Completion` struct, so that cargo check can show us where we are not
tracking completions in the code. I also attached a `#[must_use]` to
`IOResult` so that we can see the places that we are not propagating or
waiting for I/O, demonstrating locations where functions should be
reentrant and are not.
Also, while we are with this refactor in progress I want to relax the
Clippy CI lint on unused_variables.
Closes#2309
8adc807cd7