There are two reasons why we don't want a generic ra_progress crate
just yet:
*First*, it introduces a common interface between separate components,
and that is usually undesirable (b/c components start to fit the
interface, rather than doing what makes most sense for each particular
component).
*Second*, it introduces a separate async channel for progress, which
makes it harder to correlate progress reports with the work done. Ie,
when we see 100% progress, it's not blindly obvious that the work has
actually finished, we might have some pending messages still.
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