Remove the default bound from Packable::Visitor #342

Job	Run time
format	12s
build and test (ubuntu-latest, nightly)	58s
missing-files	4s
build and test (ubuntu-latest, beta)	54s
build and test (ubuntu-latest, stable)	1m 34s
build and test (macos-latest, nightly)	6h 0m 1s
build and test (macos-latest, beta)	1m 54s
build and test (macos-latest, stable)	3m 30s
build and test (windows-latest, nightly)	2m 24s
build and test (windows-latest, beta)	2m 38s
build and test (windows-latest, stable)	2m 28s
clippy	43s
udeps	6m 21s
docs	47s
licenses	2m 15s
no_std	0s
miri	0s
	6h 26m 43s

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