I want to be able to attach extra data to each `Completion`, but not
burden callers with the need to construct it. This commit helps get us
to that point by requiring callers to use a `CompletionBuilder` for
construction instead of a `Completion` itself.
I think this will also help in the future if it proves to be the case
that we can improve performance by delaying work until we actually build
a `Completion`, which might only happen if we know we won't throw it
out. But we aren't quite there yet.
This also lets us tighten things up a little bit and makes completion
construction less noisy. The downside is that callers no longer need to
consider "every" completion field.
There should not be any behavior changes here.
e9cc2f6f42