improve ergonomics of watcher utils

[clux]

So that rather than doing stuff like:

    let mut applies = try_flatten_applied(watcher(events, lp)).boxed();

    while let Some(event) = applies.try_next().await? {
        handle_event(event)?;
    }

we could do things more like:

    let mut applies = watcher(events, lp).applies();

    while let Some(event) = applies.try_next().await? {
        handle_event(event)?;
    }

The key helpers involved here are:

• utils::try_flatten_applied
• utils::try_flatten_touched

There are two things we could do:

• rename these (we kind of agree that the names are hard to grok)
• exposing these as part of a public stream extension trait

So, not sure we should do this, but it's a lot of mandatory things to import when using watchers or reflectors, trying to find a slightly nicer setup.

EDIT: Restructured this issue a bit to reflect the uncertainty in direction.

[clux]

This was done in 0.72 via #899