Require all `Listener`s to implement `Debug`. #239

Job	Run time
miri	1m 47s
build (ubuntu, stable, locked, true)	9m 9s
no_std	34s
build (windows, stable, locked, false)	14m 43s
build (macos, stable, locked, false)	12m 41s
build (ubuntu, nightly, locked, false)	8m 36s
build (ubuntu, beta, locked, false)	10m 10s
build (ubuntu, stable, latest, false)	9m 17s
build (ubuntu, stable, minimal, false)	9m 0s
deploy	4s
fuzz	15m 14s
features	23m 54s
	1h 55m 9s

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